monsoon related variations in zaire congo sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial atlantic during the last 200 000 years two sediment cores from the zaire fan and the angola margin have been investigated for their composition of terrigenous and biogenic constituents respectively for the late quaternary kaolinite feldspar ratios and variations of terrigenous element ratios of zr ti k rb and al reveal that the composition of zaire river sediment load has fluctuated in tune with precessional variations of boreal summer insolation in particular the correspondence of high kaolinite feldspar and al k ratios with low latitude insolation maxima strongly corroborates the assumption that west african monsoonal precipitation and chemical weathering was enhanced during periods of increased central african heating the most striking feauture observed is that opal accumulation has been 2 to 10 times higher in zaire fan sediments than outside in continental margin sediments off angola although biogenic ba and corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the zaire river plume and off angola from this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the zaire river plume than in the oceanic upwelling regime farther to the south off angola over the last 200 000 years the pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the zaire river to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold more arid glacial climate conditions we presume that paleoproductivity off the zaire was controlled by the following two sources of nutrients 1 marine nutrients including nitrate and phosphate as well as the uptake of ba on particulate corg caused by upwelling and 2 silica mainly delivered by the zaire river dissolved silicate hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical atlantic at a millenial timescale

seismicity rate change before the irpinia m 6 9 1980 earthquake the stability of seismicity rate in central and southern italy was examined in two data sets from 1975 0 to 1995 0 with m 3 4 and from 1987 5 to 1996 0 with m 2 5 these are the approximate minimum magnitudes of complete reporting for the respective periods the first set was used to evaluate the possibility that the 23 november 1980 irpinia m 6 9 earthquake was preceded by precursory seismic quiescence the second was used to evaluate the conditions under which a current seismic quiescence could be detected in central or southern italy during the years before the irpinia earthquake the seismicity rate in the northern half of the rupture area and north of it was low whether this was a case of precursory quiescence or not is subject to interpretation because the background rate cannot be established for the years before 1975 if we accept the relatively constant seismicity rate in the irpinia volume during the decade after the mainshock as representative for the background rate we have a clear case of precursory quiescence lasting at least 1 3 yr up to the mainshock alternatively it can be postulated that the seismicity rate during the decade following this shock was elevated regionally because of the stress redistribution and that the low rates seen before it represent the normal background rate even if this reasoning is accepted the fact remains that a volume including the northern part of the rupture produced no m 3 4 earthquakes during 1 3 yr before the irpinia earthquake but produced 10 earthquakes during the 4 7 previous yr given these facts we favor the interpretation that the irpinia 1980 earthquake was preceded by precursory seismic quiescence in the entire data set there are three other cases of quiescence of higher significance without a mainshock following since no other mainshock with m 6 exists in the data set no missed event exists we propose that in italy precursory seismic quiescence may precede mainshocks and that it may be detected in the future by the improved catalog of the modern data set beginning in 1987 5 major magnitude scale changes give the mistaken appearance that fewer large magnitude earthquakes occur in italy now than in years before 1987 we postulate that the rate of earthquakes has not changed and that the magnitude scale should be adjusted

surface wind measurements in sequal i data from meteorological recorders at 29 37w 24 00w and 14 93w all within one degree latitude of the equator show that a relaxation event in the wind speed occurred nearly simultaneously at all three during late march 1983 during sequal i wind stress subsequently increased by approximately a factor of two the event was not recognizable in the data from a fourth recorder at 6 10n 27 88w

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

testing for the predictability of dynamically triggered earthquakes in the geysers geothermal field abstract the geysers geothermal field is well known for being susceptible to dynamic triggering of earthquakes by large distant earthquakes owing to the introduction of fluids for energy production yet it is unknown if dynamic triggering of earthquakes is predictable or whether dynamic triggering could lead to a potential hazard for energy production in this paper our goal is to investigate the characteristics of triggering and the physical conditions that promote triggering to determine whether or not triggering is in anyway foreseeable we find that at present triggering in the geysers is not easily predictable in terms of when and where based on observable physical conditions however triggered earthquake magnitude positively correlates with peak imparted dynamic stress and larger dynamic stresses tend to trigger sequences similar to mainshock aftershock sequences thus we may be able to predict what size earthquakes to expect at the geysers following a large distant earthquake

change in the probability for earthquakes in southern california due to the landers magnitude 7 3 earthquake the landers earthquake in june 1992 redistributed stress in southern california shutting off the production of small earthquakes in some regions while increasing the seismicity in neighboring regions up to the present this earthquake also changed the ratio of small to large events in favor of more small earthquakes within about 100 kilometers of the epicenter this implies that the probabilistic estimate for future earthquakes in southern california changed because of the landers earthquake the location of the strongest increase in probability for large earthquakes in southern california was the volume that subsequently produced the largest slip in the magnitude 7 1 hector mine earthquake of october 1999

seasonal variability from a model of the tropical atlantic ocean abstract a numerical model incorporating a single baroclinic mode and realistic coastline geometry is used to analyze the linear dynamic response to estimates of the seasonal wind field over the tropical atlantic ocean the forced periodic response consists of a spatially dependent combination of a locally forced response kelvin waves rossby waves and multiple wave reflections the seasonal displacements of the model pycnocline are compared with observed dynamic height annual and semiannual fluctuations dominate the seasonal signal throughout the basin in general the distribution of amplitude and phase are similar for annual changes in dynamic height and pycnocline depth major features of the seasonal response are reproduced e g east west changes in pycnocline depth about a nodal point at the equator the seasonal pycnocline movement along the northern and southern coast of the guinea gulf and a significant changes of phase in the ocean variability north and south of the itcz the relative impo

a doubling of the post perovskite phase boundary and structure of the earth s lowermost mantle a new model of the deep earth allows geophysicists to measure variations in earth s temperature near the boundary between the rocky mantle and core using a recently discovered phase change in the lowest part of the earth s mantle where atoms rearrange themselves in crystals under extreme pressure and temperature the model also explains odd features of this part of the mantle such as patchy regions that reflect seismic waves generated by large earthquakes and inferences of dense partial melting combined with observations the results imply very large variations in temperature providing further evidence for circulation of material throughout the earth s mantle the thermal structure of the earth s lowermost mantle the d layer spanning depths of 2 600 2 900 kilometres1 is key to understanding the dynamical state and history of our planet earth s temperature profile the geotherm is mostly constrained by phase transitions such as freezing at the inner core boundary or changes in crystal structure within the solid mantle that are detected as discontinuities in seismic wave speed and for which the pressure and temperature conditions can be constrained by experiment and theory a recently discovered phase transition at pressures of the d layer2 3 4 is ideally situated to reveal the thermal structure of the lowermost mantle where no phase transitions were previously known to exist here we show that a pair of seismic discontinuities observed in some regions of d can be explained by the same phase transition as the result of a double crossing of the phase boundary by the geotherm at two different depths this simple model can also explain why a seismic discontinuity is not observed in some other regions and provides new constraints for the magnitude of temperature variations within d

the establishment of a pelagic sargassum population in the tropical atlantic biological consequences of a basin scale long distance dispersal event abstract starting in 2011 coastal areas of the caribbean sea and tropical atlantic ocean began to experience extraordinary yearly accumulations of pelagic sargassum brown alga historical reports place large quantities of sargassum only in the north atlantic mostly in the gulf of mexico and the sargasso sea accumulations of sargassum in the tropical atlantic have continued we used a numerical particle tracking system wind and current reanalysis data drifting buoy trajectories and satellite imagery to determine the origin of the sargassum that is now found persistently in the tropical atlantic our analyses suggest that during the extreme negative phase of the winter 2009 2010 north atlantic oscillation nao unusually strong and southward shifted westerly winds explain the transport of sargassum from the sargasso sea 20 40 n 80 20 w into the far eastern north atlantic our hindcast sargassum distributions agree with surface current simulations with the inclusion of windage windage is the additional wind induced drift of material floating at the free surface resulting from direct wind forcing on the sea surface as well as on floating or partially submerged objects in our simulations windage is included as an added vector speed and direction to the model computed surface ocean currents equivalent to 1 of surface wind velocities lagrangian analysis of the regional circulation suggests that 1 part of the sargassum subsequently drifted to the southwest in the north equatorial current nec and entered the central tropical atlantic arriving in the caribbean by the spring of 2011 with 2 another portion continuing southward along the coast of africa in the canary current eventually joining the seasonally varying system of tropical atlantic currents and thereby delivering a large sargassum population to the tropical atlantic since then sargassum patches aggregate from march to september in massive windrows along the inter tropical convergence zone itcz under the action of converging winds the windrows follow the itcz in its seasonal northward migration in the central tropical atlantic they are stretched across the central tropical atlantic as the itcz crosses the latitude of the seasonal formation of the north equatorial counter current necc these patches and windrows are exposed to high sunlight and open ocean upward flux of nutrients due to eddy and wind driven mixing in the central tropical atlantic during the northern spring and summer as the sargassum drifts farther north with the itcz large portions of the population are advected into the eastern caribbean sea some of these patches remain dispersed as the itcz migrates southward and re aggregate into new windrows as the itcz intensifies the following march april if wind mixing is strong and the mixed layer is deeper than about 50 60 m in the southern tropical atlantic at this time the sargassum will bloom and form a massive windrow otherwise the bloom will be inhibited the extreme 2009 2010 nao wind anomaly could be considered as triggering a biosphere tipping point that caused important ocean scale ecosystem changes in the tropical atlantic with significant recurrent social and economic consequences understanding whether this new expanded geographic range of massive sargassum blooms is temporary or whether it will revert to its pre 2009 distribution requires sustained monitoring and research

consistency of teleseismic reporting since 1963 abstract changes in the rate of occurrence of smaller events have been recognized in the rupture zones of upcoming large earthquakes in several postearthquake and one preearthquake study a data set in which a constant portion of the events in any magnitude band are consistently reported through time is crucial for the recognition of seismicity rate changes which are real related to some process change in the earth such a data set is termed a homogeneous data set the consistency of reporting of earthquakes in the noaa hypocenter data file hdf since 1963 is evaluated by examining the cumulative number of events reported as a function of time for the entire world in eight magnitude bands it is assumed that the rate of occurrence of events in the entire world is roughly constant on the time scale examined here because of the great size of the worldwide earthquake production system the rate of reporting of events with magnitudes above m b 4 5 has been constant or increasing since 1963 significant decreases in the number of events reported per month in the magnitude bands below m b 4 4 occurred during 1968 and 1976 these decreases are interpreted as indications of decreases in detection of events for two reasons first they occur at times of constant rates of occurrence and reporting of larger events second the decrease during the late 19609s has also been recognized in the teleseismic data reported by the international seismological centre isc this suggests that the decrease in the number of small events reported was related to facets of the earthquake reporting system which the isc and noaa share the most obvious candidate is the detection system during 1968 detection decreased in the united states central and south america and portions of the south pacific this decrease is probably due to the closure of the vela arrays bmo tfo cpo ubo and wmo during 1976 detection decreased in most of the seismically active regions of the western hemisphere as well as in the region between kamchatka and guam the cause of this detection decrease is unclear these detection decreases seriously affect the amount of homogeneous background period available for the study of teleseismic seismicity rate changes if events below the minimum magnitude of homogeneity are eliminated from the teleseismic data sets the resulting small numbers of events render many regions unsuitable for study many authors have reported seismicity rate decreases as possible precursors to great earthquakes few of these authors have considered detection decreases as possible explanations for their results this analysis indicates that such considerations cannot be avoided in studies of teleseismic data

locked and creeping patches along the hayward fault california the results of mapping locked and unlocked patches on the hayward fault using the local recurrence time method are compared with models derived from surface creep and geodetic measurements the data used were the m 1 0 earthquakes in the usgs catalog since 1981 0 located within 5 km of the fault trace along the central and southern segments agreement with the other methods is satisfactory the 1868 rupture except for its two ends is classified as a locked segment by our method in the northern part of the hayward fault the data are poor and the results of the three methods differ short local recurrence times in the northern part suggest that an asperity exists there which is capable of an m6 5 class event

evidence of remote forcing in the equatorial atlantic ocean abstract an analysis of sea surface temperature sst and surface winds in selected areas of the tropical atlantic indicates that the nonseasonal variability of sst in the eastern equatorial atlantic gulf of guinea is highly correlated with the nonseasonal variability of the zonal wind stress in the western equatorial atlantic a negative positive anomaly of the zonal wind stress near the north brazilian coast is followed by a positive negative sst anomaly in the gulf of guinea about one month later furthermore the correlation between the local wind stress anomaly and sst anomaly in the gulf of guinea is considerably smaller these preliminary results indicate that remote forcing in the western equatorial atlantic ocean is an important factor affecting the eastern equatorial atlantic sea surface temperature recent equatorial theories are consistent with these observations

winds at st peter and st paul rocks during the first sequal year wind observations were obtained during the first nine months of the sequal field program from a station maintained at st peter and st paul spp rocks winds are very steady from may to november and very unsteady during april when the intertropical convergence zone itcz crosses the site wind stress components are computed and comparison with 1979 data shows that the onset of the winds occurred one month earlier in 1983 but is of similar amplitude the spectral analysis of the wind shows a significant increase in the variance for periods between 3 5 days and 10 15 days

the implications of regional microseismic activities a lesson from 2008 ms 8 0 wenchuan earthquake abstract the ms 8 0 wenchuan earthquake has greatly perturbed the crustal stress in longmenshan fault zone lfz and its neighboring regions and hence significantly changed the regional seismicity pattern convectional coulomb stress change model based on half space dislocation source of layered elastic or viscoelastic crust can only provide brief reference to estimate the after shock sequences roughly in this study we apply the z test statistical method to examine the earthquake catalogue five years before and after this event to detect the changes of regional seismicity intensity and their statistical significance in lfz and its neighboring regions we analyze changes of the z values with respect to time and seismic magnitude in each region our results reveal that after the wenchuan earthquake seismicity intensity has significantly increased in eastern kunlun fault southern longmenshan fault longquanshan fault and huayingshan fault in sichuan basin but has significantly decreased in northern and central longmenshan fault vicinity of the songpan minjiang fault and fubianhe fault in bayankala block respectively seismicity rate change in northern and central anninghe zemuhe fault zone is not correlated with the wenchuan earthquake these results remain within 99 confidence level of the z test statistical method we compare our statistical analysis results with calculations of coseismic coulomb failure stress change due to the wenchuan earthquake from previous studies in most of the regions and fault zones the sign of the seismicity rate change agrees with the sign of the coulomb stress change

basin wide thermocline displacements along the equator of the atlantic in 1983 observations from five inverted echo sounders deployed between 10 w and 38 30 w along the equator during the spring and summer of 1983 are presented the annual development of the eastward acting zonal pressure gradient is described and related to the observed zonal wind stress the minimum and maximum basin wide gradients are observed in mid april and in the latter part of july respectively the upwelling season in the gulf of guinea is identified at the easternmost sounder during may june and july pulses in thermocline displacement first deepening then shallowing propagate eastward across the basin at speeds of 2 8 and 2 1 m s 1 they arrive at 10 w at the onset and just after the cessation of the seasonal upwelling at that location their initialization is related to relaxation periods in the zonal wind for four of the last five years the time of the pulse preceding the upwelling is tentatively identified

seasonal heat transport in a forced equatorial baroclinic model abstract seasonal heat transport is examined in a simple linear shallow water model on the equatorial beta plane it is found in this model that meridional transport by the seasonally varying western boundary current is of the same magnitude but opposite phase to the seasonally varying interior transport and therefore tends to cancel

response of the san andreas fault to the 1983 coalinga nunez earthquakes an application of interaction based probabilities for parkfield 1 the parkfield cholame section of the san andreas fault site of an unfulfilled earthquake forecast in 1985 is the best monitored section of the world s most closely watched fault in 1983 the m 6 5 coalinga and m 6 0 nunez events struck 25 km northeast of parkfield seismicity rates climbed for 18 months along the creeping section of the san andreas north of parkfield and dropped for 6 years along the locked section to the south right lateral creep also slowed or reversed from parkfield south here we calculate that the coalinga sequence increased the shear and coulomb stress on the creeping section causing the rate of small shocks to rise until the added stress was shed by additional slip however the 1983 events decreased the shear and coulomb stress on the parkfield segment causing surface creep and seismicity rates to drop we use these observations to cast the likelihood of a parkfield earthquake into an interaction based probability which includes both the renewal of stress following the 1966 parkfield earthquake and the stress transfer from the 1983 coalinga events we calculate that the 1983 shocks dropped the 10 year probability of a m 6 parkfield earthquake by 22 from 54 22 to 42 23 and that the probability did not recover until about 1991 when seismicity and creep resumed our analysis may thus explain why the parkfield earthquake did not strike in the 1980s but not why it was absent in the 1990s we calculate a 58 17 probability of a m 6 parkfield earthquake during 2001 2011 index terms 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics 7260 seismology theory and modeling keywords coalinga earthquake parkfield stress change earthquake probability seismicity rate

reconstruction of past nutrient utilization in the eastern angola basin based on sedimentary 15n 14n ratios the 15n records of two sediment cores from the angola basin have been used to investigate past changes in nutrient utilization in the surface waters of this highly productive region low 15n 14n ratios in the cores from 3000 m water depth generally correspond to low sea surface temperatures and high sedimentary organic content demonstrating a link between upwelling productivity and the degree of nitrate depletion glacial 15n was lower by 0 5 0 7 and paleoproductivity was elevated by around 30 relative to interglacial periods indicating that although productivity was higher nitrate was less depleted during glacial than interglacial periods more pronounced than the glacial interglacial changes in 15n values of both cores are precession related 23 kyr fluctuations these cyclic 15n variations which are in phase with those of paleoproductivity and sea surface temperature indicate that changes in trade wind driven upwelling intensity and the advection of cold nutrient rich water from the south drive nutrient availability and productivity off the southwest coast of africa no strong evidence was found from sedimentary 15n values for denitrification in the water column in the angola basin during the past 180 kyr

in situ wind measurements and the ocean response in the equatorial atlantic during the programme francais ocean et climat dans l atlantique equatorial and seasonal response of the atlantic ocean experiment in situ wind measurements collected as part of the programme francais ocean et climat dans l atlantique equatorial focal seasonal response of the equatorial atlantic sequal experiment 1983 1984 in the western and eastern parts of the equatorial atlantic basin are described they were obtained from meteorological stations placed saint peter peter and saint paul rocks spp 1 n 29 w and at the top of a surface buoy moored in the gulf of guinea 0 n 4 w from the wind observations the wind stress was inferred and results are compared with climatology the wind stress time series show the abrupt increase of the winds during the spring that at spp reaches a value as high as 0 35 dyn cm2 in 2 weeks for both observed years the 11 day running mean time series shows that the onset of the zonal component of the wind stress occurs at spp on april 10 1983 and on may 17 1984 and in the gulf of guinea gg on april 5 1983 and april 10 1984 the monthly mean observations show an interannual variability both in the time of the onset and in the strength of the trade winds at spp and gg the total wind stress increases 1 month earlier than climatology in 1983 but at the same time as climatology in 1984 at spp the zonal component of the wind stress also intensifies 1 month earlier than climatology in 1983 but 1 month later in 1984 the equatorial temperature records at 28 w and 4 w show that the depth of the 20 c isotherm on a seasonal time scale decreases during the relaxation period of the trade winds boreal winter in 1983 1984 this occurred in december 1983 at 28 w and in march 1984 at 4 w after the onset of the local trade winds the thermocline continues to move upward during 1 month at 28 w and during 3 months at 4 w thereafter the thermocline deepens at both locations at the surface the temperature decreases when the trade winds intensify and remains low as long as the trade winds are blowing the seasonal variations of the temperature both at the surface and below the surface at 28 w and 4 w are interpreted in the light of the results of a nonlinear multilevel model in the cases of a sudden increase and a sudden relaxation of the trade winds

seismic b value anomalies in the sumatran region seismotectonic implications abstract the spatial variations in the seismic b value seismic moment bouguer gravity change and fault plane solution are estimated in the sumatran region based on homogeneous seismicity data from june 2005 to march 2012 to investigate the seismotectonic characteristics and tectonic activity between the 2004 sumatran earthquake and the 2012 east indian ocean earthquakes based on changes in the seismicity rate this time period is divided into five intervals with different earthquake distributions and faulting styles the seismic b values faulting mechanisms and seismic moment release values are mapped throughout the sumatran region and the gravity change every two years between the 2004 event and the 2012 events are determined from satellite grace gravity data the b values vary from 0 45 to 1 59 and an inverse correlation is observed between the b values and convergence rates low b values are located at depths between 20 km and 40 km in the subduction zone and between 20 km and 35 km in the oceanic plate the lowest b values are located at a depth of approximately 28 km in both regions which indicates a homogeneous fault system between them the areas with low b values are associated with megathrust and strike slip mechanisms while the area with the lowest b values is dominated by a megathrust mechanism seismically active regions have low b values associated with a high stress accumulation and high seismic moment release low b value areas are located mainly along the andaman sunda trench and andaman nicobar sumatra forearc temporal variations in the seismicity distribution and faulting style are recognized with temporal variations in the b values in each period relationships exist between the epicenter of the 2004 event and the location of the maximum bouguer gravity change in 2012 and between the epicenters of the 2012 events and the location of the maximum bouguer gravity change in 2004 furthermore a zone of continuous 2 year positive gravity change is located near the nicobar islands the 2004 event boost the 2012 events and intraplate seismicity via stress transfer the seismicity distributions in individual periods within the studied timeframe highlight the active and locked portions of the fault systems

response of equatorial oceans to periodic forcing oscillating wind with a period p induce variability with the following characteristics in the upper few hundred meters of the equatorial zone 5 sup 0 n to 5 sup 0 s of the ocean 1 p 10 days these winds fluctuate too rapidly to generate strong currents and excite primarily waves 2 10 days 150 days the amplitude of variability is almost independent of frequency an equilibrium response which is in phase with the forcing and which corresponds to a succession of steady states is approached asymptotically these time scales are for a basin 5000 km wide if the width of the basis exceeds 5000 km then the 150 daytime scale increases in the deep ocean below the thermocline motion corresponds to propagating waves generated by the divergence of the nonlinear currents in the upper ocean

large occurrence patterns of new zealand deep earthquakes characterization by use of a switching poisson model the paper studies the statistical properties of deep earthquakes around north island new zealand we first evaluate the catalogue coverage and completeness of deep events according to cusum cumulative sum statistics and earlier literature the epicentral depth and magnitude distributions of deep earthquakes are then discussed it is worth noting that strong grouping effects are observed in the epicentral distribution of these deep earthquakes also although the spatial distribution of deep earthquakes does not change their occurrence frequencies vary from time to time active in one period relatively quiescent in another the depth distribution of deep earthquakes also hardly changes except for events with focal depth less than 100 km on the basis of spatial concentration we partition deep earthquakes into several groups the taupo bay of plenty group the taranaki group and the cook strait group second order moment analysis via the two point correlation function reveals only very small scale clustering of deep earthquakes presumably limited to some hot spots only we also suggest that some models usually used for shallow earthquakes fit deep earthquakes unsatisfactorily instead we propose a switching poisson model for the occurrence patterns of deep earthquakes the goodness of fit test suggests that the time varying activity is well characterized by a switching poisson model furthermore detailed analysis carried out on each deep group by use of switching poisson models reveals similar time varying behavior in occurrence frequencies in each group

seasonal cycle in the upper equatorial atlantic ocean the dynamics of the seasonal cycle in the upper equatorial atlantic ocean are studied using observations and a hierarchy of ocean models distinctive features of the seasonal cycle are strong annual and semiannual components eastward westward propagating sea surface height ssh and thermocline depth at the equator off the equator and westward propagating surface zonal currents at the equator modelling results show that linear theory can explain the seasonal cycle in thermocline depth and ssh while first order linear theory can also explain the structure of the seasonal cycle of surface zonal currents at the equator nonlinear terms are required they weaken the variability and improve its phase and zonal extent the important terms are meridional and vertical advection and vertical diffusion of zonal momentum the linear solution is essentially determined by the four gravest baroclinic modes and kelvin and first meridional mode rossby waves the eastward propagation in thermocline depth at the equator results from the kelvin wave contribution while the westward propagation in thermocline depth off the equator and surface zonal currents at the equator result from the first meridional mode rossby wave the contribution of kelvin and rossby waves generated by boundary reflections equals that of the directly forced waves the semiannual cycle in zonal winds although much weaker than the annual component forces a strong semiannual component in ssh and surface zonal currents because it excites the basin mode of the second baroclinc mode this explains the observed feature in the seasonal cycle from march to august

inadvertent changes in magnitude reported in earthquake catalogs their evaluation through b value estimates a simple procedure is presented for analyzing magnitudes and seismicity rates reported in earthquake catalogs in order to discriminate between inadvertently introduced changes in magnitude and real seismicity changes we assume that the rate and the frequency magnitude relation of the independent background seismicity do not change with time observed differences in the frequency magnitude relation a and b values between data from two periods are modeled as due to a transformation of the magnitude scale the transformation equation is found by a least squares fitting process based on the seismicity data for earthquakes large enough to be reported completely and by comparing the linear relation of one period to the other for smaller events an additional factor accounting for increased decreased detection is allowed this fitting technique is tested on a data set from parkfield for which two types of magnitudes amplitude and duration were computed for each earthquake we found that the b value fitting technique yielded virtually the same result as a linear regression assuming the same errors in the two magnitudes the technique is also applied to interpret the nature of reporting rate changes in a local guerrero mexico and a regional italy earthquake catalog in guerrero a magnitude change in 1991 37 can be modeled about equally well by m new m old 0 5 or by m new 1 02 m old 0 38 but residuals with the latter transformation are smaller in italy a magnitude change in 1980 21 cannot be modeled satisfactorily by a simple magnitude shift but is well described by a compression of the magnitude scale given by m new 0 67 m old 1 03 the proposed b slope fitting method provides a means to interpret quantitatively and in some cases correct for artificial reporting rate changes in earthquake catalogs

some premonitory phenomena of the 1995 hyogo ken nanbu kobe earthquake seismicity b value and fractal dimension abstract this paper presents a detailed and complex analyses of the spatio temporal evolution of seismic activity between 1976 and 1998 in a broad area surrounding the epicentre of the 1995 hyogo ken nanbu kobe earthquake as well as in the source area various precursory changes in seismicity such as quiescence followed by increased seismic activity b value and fractal dimension correlation dimension d2 anomalies appear approximately two years before the occurrence of the major event an increased level of background seismicity is clearly observed after the occurrence of the kobe earthquake particularly in the northeast extension of the source area all these anomalous changes in seismic activity before and after the occurrence of the 1995 hyogo ken nanbu earthquake appear in a relatively large region surrounding the epicentre of the main shock as well as in the source area

velocity and temperature observations during the seasonal response of the equatorial atlantic experiment at 0 28 w continuous 2 7 year time series of upper ocean currents and temperature at 0 28 w collected during the seasonal response of the equatorial atlantic experiment are presented the thermocline underwent distinctive annual cycles in response to forcing by the surface wind stress and the core of the equatorial undercurrent tracked the thermocline while the vertical position of the thermocline and the undercurrent appear to be simply related the transport of the undercurrent did not show a replicating annual cycle and the speed at the undercurrent core remained relatively constant near surface flow when averaged over the 2 7 years was not statistically different from zero and a westward flowing south equatorial current was rarely established despite strong easterly wind stress in all 3 years cross equatorial flow was also zero in the mean at this location

robust and exploratory analysis of active mesoscale tectonic zones in japan utilizing the nationwide gps array abstract a monitoring gps array recently developed in japan can yield nationwide maps of active inland tectonic zones atzs on a mesoscale approximately 70 to several hundred kilometers in lateral extent but it has been difficult to characterize atzs in japan as they are in fact operational on multiple scales and our efforts are often hindered by various irregularities in the data the key to overcoming these problems would be to gain an insight into the available data before any precise kinematic modeling is performed with indefinite assumptions in this study horizontal velocity fields deduced from the nationwide gps array were treated with a set of techniques in robust smoothing and exploratory data analysis that brought out exceptionally powerful mesoscale atzs and made them easier to characterize the resolved atzs were then retrospectively monitored to study their regional and temporal variations using a set of approx 840 observation stations about 30 km apart for a 4 year series of fixed observation time intervals 810 days each the smoothing operation involved three steps 1 imputation of the velocity fields for the purpose of anti aliasing 2 robust smoothing of the velocity fields with the median operative and 3 visualization of deformation rate distributions in several coordinate independent parameters and post filtering the geometrical resolvability of mesoscale atzs was confirmed by calibrating the smoothing scheme against synthetic tectonic boundary models before it was applied to the case study in japan atzs in japan which are essentially visible as systematic deviations in the velocity fields on the international terrestrial reference frame itrf and as strain rate anomalies were highlighted sharply along some known tectonic zones chains of active volcanoes and areas above low seismic velocity anomalies in the crust and upper mantle all of which generally paralleled the offshore trench axes the geometrical agreements among the mapped atzs and the physical anomalies in the crust are presumably due to their common structural weakness on the mesoscale in the four main islands of japan all but 30 40 of the strain rate anomalies persisted during the entire 6 years of the case study period while the rest sporadically appeared or disappeared in a period from several months to a few years the transient shifts in the deformation rates were remarkably synchronous with some nearby major tectonic episodes large earthquakes and slow events differential plate coupling strengths along the subduction zones can also be inferred from the persistent pattern of rotational strain rate anomalies forming clockwise and counterclockwise pairs along the pacific our empirical observations suggest that the first order features of interseismic crustal deformations in japan can be characterized as collateral processes behaving in response to fluctuations of the tectonic stresses on multiple scales likely influenced by changes of plate coupling strengths on the contiguous subduction faults

methods for measuring seismicity rate changes a review and a study of how the m w 7 3 landers earthquake affected the aftershock sequence of the m w 6 1 joshua tree earthquake the development of fault interaction models has triggered the need for an accurate estimation of seismicity rate changes following the occurrence of an earthquake several statistical methods have been developed in the past to serve this purpose each relying on different assumptions e g stationarity gaussianity pertaining to the seismicity process

seasonal adjustments in the equatorial atlantic during 1983 as seen by surface moorings upper ocean seasonal variations in velocity and temperature observed with current meters on surface moorings along the equator in the atlantic during february september 1983 are shown the longitudinal variation in the duration of upwelling and downwelling responses to zonal wind stress changes are discussed these responses exhibit an integral character dependent upon longitudinal position which is consistent with forced linear long wave theory

on the baroclinic response of the zonal pressure gradient in the equatorial atlantic ocean temperature time series obtained from surface moorings deployed along the equator in the atlantic ocean during the seasonal response of the equatorial atlantic experiment and the programme francais ocean et climat dans l atlantique equatorial were used to investigate the baroclinic response of the zonal pressure gradient due to the sloping isopycnals the zonal pressure gradient evolved as a complicated function of time depth and zonal position the most rapid and spatially inhomogeneous variations corresponded to periods of rapidly changing easterly wind stress following these periods the baroclinic response due to the sloping isopycnals was observed to overshoot its intended equilibrium before relaxing to it in so doing it tended to buffer pressure gradient variations at the depth of the equatorial undercurrent euc over much of the year the isopycnal slopes opposed the surface slope after the winds relaxed in winter however the isopycnal slopes reversed and acted in concert with the surface slope thereby enhancing the pressure gradient at the depth of the euc relative to that at the surface this reversal might be important in maintaining the euc during times of weak easterly wind stress the vertical structure of the zonal pressure gradient varied with the depth of the thermocline when the winds were weak and the thermocline was shallow the baroclinic response of the zonal pressure gradient due to the sloping isopycnals was primarily effected in the upper 50 m after the winds intensified and the thermocline deepened the baroclinic response extended down to at least 150 m

equatorial atlantic ocean temperature and current variations during 1983 and 1984 the equatorial regions of the earth s oceans are climatically sensitive ones because of the zonal sea surface temperature contrasts observed there1 equatorial sea surface temperature normally varies on an annual cycle with the prevailing trade winds but deviations from this cycle may have significant global implications as occurred for example during the 1982 83 pacific ocean el nino southern oscillation event2 understanding the annual and interannual variability of the tropical oceans has therefore been the goal of several measurement programmes in the atlantic ocean the seasonal response of the equatorial atlantic sequal experiment and the programme francais ocean et climat dans l atlantique equatorial focal have provided a basin wide and synoptic data set over two annual cycles we present here results from surface moored current meters which were one of several fixed and shipborne measurement systems employed by sequal and focal we will describe the evolution of the upper ocean thermal and zonal velocity component variations in relation to forcing by the trade winds show differences observed along the equator at 28 w and 4 w and compare the oceans responses at these locations during 1983 and 1984 the synoptic data realizations of these years differed from climatology and these differences are related to the rapidly varying nature and intensity of the wind stress in a given year changes in wind stress from year to year result in interannual variability as a modulated annual cycle and 1984 a year of weak winds relative to 1983 offers a case in point the zonal sea surface temperature gradient vanished along the equator in 1984 during the season when it normally would have been a maximum

two current seismic quiescences within 40 km of tokyo summary two highly significant seismic quiescence anomalies that started in 1993 0 exist currently within about 40 km of tokyo they are located in central saitama province at 35 99 n 139 56 e and in tokyo bay south of funabashi at 35 59 n l39 94 e the radii of the anomalous volumes are approximately 15 km centred at about 25 and 20 km depth respectively the seismicity rate during the quiescence was less than 10 per cent background rate in both cases the significance of these quiescences measured by the standard deviate z test comparing the rate of the last three years to the backgound rate is higher than for any other rate changes present in the catalogue zmax 7 5 and 7 2 respectively however episodes of highly significant quiescences which could have been interpreted as precursors but were not followed by main shocks also exist in the data set a third seismic quiescence was found to be in progress since 1993 3 0 5 in nw saitama province at 36 18 n 139 22 e it has a radius of 14 km and is less significant than the other two anomalies zmax 6 8 in a polygon that covers the areas of all three quiescences defined above the seismicity rate since 1994 is only 30 per cent of the previous background rate i e 100 earthquakes that would normally have occurred did not this analysis is based on the earthquake catalogue of the national institute for earth science and disaster prevention for the years 1980 1996 2 and mmin 1 0 as well as for the years 1986 2 1996 2 with mmin 0 1 the anomalies are equally clear in the clustered and in the full catalogues to avoid contamination by explosions only nighttime data 19 00 to 7 00 hours local time were used the frequency magnitude distributions in the quiescent volumes at central saitama nw saitama and funabashi have b values of 1 3 1 5 and 1 1 respectively based on the hypothesis that crustal volumes with high b values may not be capable of m6 earthquakes these three quiescent volumes may be ruled out as source volumes for main shocks of these magnitudes using lilt seismic quiescence hypothesis alone it is a reasonable interpretation to suggest that m 6 5 0 5 earthquakes are expected to terminate each of the three quiescences within 1 year 1 year from now with a 50 per cent probability each the interpretation that all three quiet volumes could rupture together in a large earthquake seems less likely however based on the information that the quiescences are located in volumes with b values above the average b 0 8 it can be argued that the three current quiescences are false alarms the fact that these two lines of evidence allow opposing interpretations in these cases shows that neither hypothesis is sufficiently tested thus the quiescence hypothesis is not advanced enough for issuing earthquake predictions nevertheless it is important that scientific tests of prediction scenarios are formulated in real time so we can learn how to modify our hypotheses to make them useful for prediction in the future

late quaternary paleoproductivity changes off the congo deduced from stable carbon isotopes of planktonic foraminifera a detailed record 2 kyr intervals of the difference in stable carbon isotopes 13c between globigerina bulloides and globigerinoides ruber pink is used to reconstruct changes in upwelling intensity off the congo river for the last 190 000 yr comparisons of the oxygen and carbon isotope data from this core with records from the niger fan and from pelagic cores in the eastern equatorial south atlantic indicate that the congo fan isotope records do not contain a strong freshwater signal as is described off other major rivers the temporal pattern of the 13c signal correlates with the marine organic carbon record from the congo fan thus the planktonic 13c record reflecting past changes in upwelling intensity and nutrient content corroborates the signal provided by sedimentary organic carbon which is presumed to indicate changes in the amount of biological productivity and export flux to the seafloor the planktonic 13c signal is characterized by a dominant 23 kyr periodicity which provides evidence for a strong response of upwelling fluctuations off the congo to precessional forcing minima in the 13c record are aligned with periods of minimum boreal summer insolation over central africa reflecting an increase of upwelling and biological productivity off the congo at periods of enhanced zonal intensity of southeast trades and corresponding weak southeast monsoon over the eastern south atlantic a strong response to changes in river discharge probably did not occur indicating that fertilization by river derived nutrients has played only a minor role with respect to late quaternary changes in the total amount of primary productivity off the congo

seasonal variations of the subsurface thermal structure in the gulf of guinea abstract the subsurface thermal structure in the gulf of guinea is analyzed using the historical hydrographic data file of particular interest is the rapid vertical displacement of the thermal structure from the warm march may season to the cold july august season this displacement field is trapped at the equator with vertical and meridional scales appropriate to a second or third mode baroclinic kelvin wave it has a maximum amplitude between 4 w and 10 w with a node near 2 e south of 4 s no seasonal displacement of the thermocline underlies the seasonal variation in the sea surface temperature along the eastern boundary north of the equator to 3 e the displacement field vanishes thus the coastal upwelling associated with a vertical displacement tightly bound to the northern coast from 2 e to 10 w appears detached from the equatorial regime

propagation of the seasonal upwelling in the eastern equatorial atlantic abstract several mechanisms have been proposed to explain the coastal and equatorial upwelling in the eastern atlantic guinea gulf the most controversial is the mechanism of remote wind forcing in the western equatorial atlantic suggested by moore et al 1978 most of the possible explanations for the upwelling and their relative importance are discussed in view of recent observations detailed analysis of daily sea surface temperature sst collected at 16 coastal stations along the northern coast of the guinea gulf reveals that the upwelling event propagates westward along this coast at a mean speed of 0 7 m s 1 similar analysis of historical monthly mean sst data shows that the coastal upwelling event propagates poleward from 1 s to at least 13 s at the same phase speed furthermore the northern hemisphere and southern hemisphere coastal upwelling signals seem to start at the same time from the equator the same kind of analysis applied to hydrographic data from a station situated 41 km off abid

variations of b values at the western edge of the ryukyu subduction zone north east taiwan using earthquakes relocated in north east taiwan we estimated b value distribution along a cross section located near the ryukyu slab edge and four b value anomalous areas are evidenced 1 a high b value body lying on top of a low vp low vs and high vp vs sausage like body was considered as a region of enhanced partial melt or water supply above which seismicity occurs 2 beneath the ilan plain an anomalous area characterized by b values slightly higher than 1 1 might give evidence to the magma conduits to the kueishantao island 3 above the ryukyu wadati benioff zone at depths ranging from 90 to 110 km a high b value anomaly might correspond to the depth where dehydration occurs in the subducting oceanic plate and 4 a low b value area located within the ryukyu slab at depths ranging from 70 to 90 km might be linked to the compressive mechanisms shown by focal mechanisms and the bending of the subducting plate

on the response of the equatorial thermocline in the atlantic ocean to the seasonally varying trade winds measurements made during the seasonal response of the equatorial atlantic experiment in 1983 when the easterly trade winds near the equator ceased and then intensified as part of their annual cycle show a sequence of events leading to an adjusted thermocline both the duration and extent of the downwelling and upwelling associated with the adjustment process were zonally inhomogeneous along the equator an analytical reduced gravity model was driven by a hierarchy of easterly wind stress forcing functions representative of the atlantic to compare the integral nature of the forced long wave responses with the observations qualitative agreement was found with upwelling and downwelling sequences within the forced region increasing to the east as observed apparent model propagation speeds differed significantly from the prescribed model wave speeds within the forced region owing to the growth parameters of the winds and boundary reflections distorted the responses to the east of the forced region the resulting interference patterns tended to separate the region 0 10 w along the equator from the african coast as one of maximum upwelling

seismicity rate changes along the central california coast due to stress changes from the 2003 m 6 5 san simeon and 2004 m 6 0 parkfield earthquakes abstract we investigated the relationship between seismicity rate changes and modeled coulomb static stress changes from the 2003 m 6 5 san simeon and the 2004 m 6 0 parkfield earthquakes in central california coulomb stress modeling indicates that the san simeon mainshock loaded parts of the rinconada hosgri and san andreas strike slip faults along with the reverse faults of the southern los osos domain all of these loaded faults except for the san andreas experienced a seismicity rate increase at the time of the san simeon mainshock the parkfield earthquake occurred 9 months later on the loaded portion of the san andreas fault the parkfield earthquake unloaded the hosgri fault and the reverse faults of the southern los osos domain which both experienced seismicity rate decreases at the time of the parkfield event although the decreases may be related to the decay of san simeon triggered seismicity coulomb stress unloading from the parkfield earthquake appears to have altered the aftershock decay rate of the southern cluster of san simeon aftershocks which is deficient compared to the expected number of aftershocks from the omori decay parameters based on the pre parkfield aftershocks dynamic stress changes cannot explain the deficiency of aftershocks providing evidence that static stress changes affect earthquake occurrence however a burst of seismicity following the parkfield earthquake at ragged point where the static stress was decreased provides evidence for dynamic stress triggering it therefore appears that both coulomb static stress changes and dynamic stress changes affect the seismicity rate

teleseismic detection in the aleutian island arc recently it has become apparent that teleseismic detection has decreased substantially in many regions of the world the major decrease was related to the closure of the vela arrays in the united states during the late 1960 s this detection decrease has been recognized in south and central america mexico the kuriles the caribbean tonga and the new hebrides in this paper the effect of the closure of these arrays on the reporting of events in the aleutian island arc is examined in the aleutians the detection history is complicated by the short term installation of a local network on and near amchitka island during the early 1970 s the temporal coincidence of the installation of this network and the closure of the vela arrays delayed the detection decrease in the central aleutians until the amchitka network was closed in early 1973 reporting in the eastern aleutians was unaffected by the installation of the amchitka network in that region the detection decreased between 1968 and 1970 the time of the closure of the vela arrays new techniques have been developed which make it possible to determine the effect of station installation or closure on the reporting in some regions these techniques rely on plots which show the distribution of an observed seismicity rate change in the magnitude domain these plots make it possible to recognize probable detection changes and to determine quantitatively magnitude cutoffs which avoid these changes the magnitude level at which these cutoffs are made is termed the minimum magnitude of homogeneity mmin h the reporting of events with mb 4 6 in the aleutians decreased substantially during the mid 1970 s so mmin h in this region is 4 7 this is different from the magnitude of completeness mmin c which is mb 5 0 0 1 if one is interested in examining seismicity rates for changes which may be precursors to earthquakes then awareness of detection related changes and magnitude cutoffs which avoid these changes are essential for meaningful results

further studies on the response of the equatorial thermocline in the atlantic ocean to the seasonally varying trade winds upper ocean temperature data obtained during the seasonal response of the equatorial atlantic experiment and the francais ocean et climat dans l atlantique equatorial program are used to describe the variations of the thermocline observed along the equator from february 1983 to september 1984 in response to rapid seasonal changes in zonal wind stress the thermocline underwent sequences of upwelling followed by downwelling or conversely depending upon whether the winds were intensifying or relaxing these sequences increased systematically in magnitude and duration to the east both the sequential behavior and its zonal inhomogeneity represent departures from the slowly varying and in phase climatological description of the annual thermocline response an analytical reduced gravity equatorial long wave model forced by zonal wind stress distributions representative of the variations observed along the equator was employed to reconcile the differences between the 1983 1984 synoptic measurements and the climatologically averaged description qualitative agreements between the model results and the data were found for many salient features of the annual cycle including the zonal pressure gradient calculated by finite difference the features in the model arise as interference patterns between directly forced and reflected kelvin and long rossby waves that are generated with each change in the wind stress the manner in which the wind stress changes from year to year is therefore crucial to the equatorial ocean s response

earthquake triggering during preparation for great earthquakes seismic quiescence has been proposed as a precursor to many large earthquakes in this paper we present two cases in which precursory quiescence is interrupted several years prior to great earthquakes by triggered events in the upcoming rupture areas this triggering results in complex precursory patterns which include seismic quiescence and triggered events prior to the kermadec earthquake of 1976 ms 8 0 events in the rupture zone were triggered by a quake on the outer rise and one near the future mainshock epicenter events were triggered by an earthquake on an adjacent segment of the plate interface and by one near the epicenter of the new hebrides earthquake of 1980 ms 8 0 we propose that two preconditions must be fulfilled for triggering to occur first the stress in the region must be close to the failure stress and second nearby events must occur which are large enough to affect the stress field but not large enough to cause an immediate mainshock

crustal heterogeneity highlighted by spatial b value map in the wellington region of new zealand summary we map the b value in the subduction zone of the wellington region new zealand using a high quality earthquake catalogue relocated with a 3 d seismic velocity model consisting of 50 314 events that occurred between 1990 and 2005 in order to investigate heterogeneity in the crust of the overlying plate and in the upper plane of the wadati benioff zone wbz we analyse a series of cross sections perpendicular to the strike of the subduction zone we calculate the b values selecting events with magnitude of completeness 2 4 and depth 65 km and projecting the seismicity within 20 km on each side of the cross sectional planes we observe areas of high b value 1 7 near the plate interface and regions of low b value anomalies are detected both in the wbz in the northwest region below 40 km depth and in the overlying plate in the northern south island at 10 km depth the anomalies are statistically significant based on utsu s p test and the bootstrap method and are not data processing method or parameter dependent we compare the b value distribution with previously determined 3 d distributions of vp vp vs and qp from seismic tomography this comparison suggests that material inhomogeneity caused by fluid filled cracks resulting from dehydration of the subducted slab and subducted sediments is the predominant cause of b value variation in the shallow part of this subduction zone our observations are consistent with a previously proposed conceptual model that fluid distribution in the shallow part of this subduction zone is controlled by the permeability of geological terranes in the overlying plate

seismic quiescence stress drops and asperities in the new hebrides arc the rate of occurrence of earthquakes shallower than 100 km during the years 1963 to 1980 was studied as a function of time and space along the new hebrides island arc systematic examination of the seismicity rates for different magnitude bands showed that events with mb 4 8 were not reported consistently over time the seismicity rate as defined by mb 4 8 events was examined quantitatively and systematically in the source volumes of three recent main shocks and within two seismic gaps a clear case of seismic quiescence could be shown to have existed before one of the large main shocks if a major asperity was excluded from the volume studied the 1980 ms 8 rupture in the northern new hebrides was preceded by a pattern of 9 to 12 yr of quiescence followed by 5 yr of normal rate this pattern does not conform to the hypothesis that quiescence lasts up to the mainshock which it precedes the 1980 rupture also did not fully conform to the gap hypothesis half of its aftershock area covered part of a great rupture which occurred in 1966 a major asperity seemed to play a critical role in the 1966 and 1980 great ruptures it stopped the 1966 rupture and both parts of the 1980 double rupture initiated from it in addition this major asperity made itself known by a seismicity rate and stress drops higher than in the surrounding areas stress drops of 272 earthquakes were estimated by the ms mb method time dependence of stress drops could not be studied because of changes in the world data set of ms and mb values areas of high stress drops did not correlate in general with areas of high seismicity rate instead outstandingly high average stress drops were observed in two plate boundary segments with average seismicity rate where ocean floor ridges are being subducted the seismic gaps of the central and northern new hebrides each contain seismically quiet regions in the central new hebrides the 50 to 100 km of the plate boundary near 18 5 s showed an extremely low seismicity rate during the entire observation period low seismicity could be a permanent property of this location in the northern new hebrides gap seismic quiescence started in mid 1972 except in a central volume where high stress drops are observed this volume is interpreted as an asperity and the quiescence may be interpreted as part of the preparation process to a future large main shock near 13 5 s

spatial variation in the frequency magnitude distribution of earthquakes under the tectonic framework in the middle east abstract spatial variations of seismic energy released and b value over the middle east region are investigated based on a seismicity catalog from 1995 to 2007 the goal is to use these seismic parameters and based on other geodetic and geophysical observations such as gps measurements strain rate model fault distribution focal mechanism crustal model q model and gravity measurements etc to uncover spatial patterns that seem anomalous areas of high energy released cumulative seem to correspond to the areas of relatively high b values areas of high energy released and high b values seem to correspond very well with the location of continental collision where earthquake activities are high the divergent boundary between arabia and african plates and subduction zone at makran seem to correspond to low to moderate energy release northern pamir azerbaijan caucasus the lower part of zagros mountains eastern turkey owen fracture zone strait of bob el mandeb and south of the sulaiman shear zone seem to correspond to high cumulative energy released high strain rate high b values and high fault density while the central and eastern iran southern zagros northern pakistan gulf of aden alborz southwest of the caspian sea western caucasus and kopeh dagh seem to correspond with lower b values the cross section map for hindu kush shows general decreasing of the b values with depth however a region of high b value is observed underneath pamir at depths from 170 to 230 km this anomaly region can be due to dehydration of pamir crustal slab at depth

evaluation of precursory seismic quiescence in sixteen subduction zones using single link cluster analysis this paper describes a new method single link cluster analysis slc to evaluate percursory quiescence for shallow earthquakes in sixteen subduction zones using data from the isc catalog to define quiescent regions we divided the catalog into time intervals with a durationt overlapping byt 2 we considered all earthquakes having magnitudes larger than some magnitudemmin lying within a specified distance of a great circle which is approximately coincident with the trench near a subduction zone within each time interval we connected or linked all earthquakes lying within some cutoff distanced of one another we then projected all these links onto the great circle and defined a region to be quiescent if it was not covered by the projection of any links for this study t was two years mmin wasmb 4 9 and we variedd from 100 to 400 km we defined an earthquake as following quiescence if it occurred within two years following and within 75 km of a quiescent zone as defined above the primary conclusion of this study was that earthquakes with surface wave magnitudes 7 2 and greater were about 5 15 more likely to follow quiescence than were the smaller background earthquakes withmb 4 9 a chi squared analysis shows that this result is significant at the 99 level in contrast earthquakes with surface wave magnitude of 6 7 to 7 1 were no more likely to follow quiescence than were background earthquakes of sixteen individual regions central america japan and peru chile were the only regions where large earthquakes were more likely to occur following quiescence than were background earthquakes for a cutoff link length of 300 km only in central america was the difference between large earthquakes and background earthquakes significant at the 95 level of significance for a cutoff link length of 250 km the significance level exceeded 95 only in japan the slc method is an objective quantitative method for evaluating large data catalogs or for monitoring quiescence in regions where quiescence is conjectured to precede large earthquakes