effects of h2o on elastic wave velocities in ultrabasic rocks at 900 c under 1 gpa simultaneous measurements of compressional and shear wave velocities vp and vs in five ultrabasic rocks with hydrous minerals were carried out from room temperature to 900 c under 1gpa in a closed system both vp and vs in all five rocks decreased markedly near 600 c and or 800 c following the dehydration of hydrous minerals in the rocks considering velocities only in the anhydrous matrix which were estimated from the volumetric ratio of forsterite and enstatite in each sample the effects of water produced by the dehydration reactions were estimated the ratio of v at 900 c under 1gpa is represented by the following equation v 1 0 0027 w to both p and s wave where v is the velocity in the rock including the extricated water w kg m 3 and is the velocity in the sample excluding the extricated water the low velocity layer lies between the descending lithosphere and the overlying asthenosphere beneath the japanese island arcs and the ratio of vp drop for the low velocity layer to that of the underlying descending lithosphere is approximately 5 if the observed velocity drop is wholly attributed to the free water produced by dehydration of serpentine minerals in the descending slab the required volumetric ratio of serpentine minerals in the descending slab would be 5 10

classification of new ice in the greenland sea using satellite ssm i radiometer and seawinds scatterometer data and comparison with ice model abstract in the ice covered waters of the greenland sea the polarisation ratio of quikscat seawinds ku band 13 4 ghz scatterometer measurements and the polarisation ratio of dmsp ssm i 19 ghz radiometer measurements are used in combination to classify new ice and mature ice in particular the formation of the new frazil pancake ice odden 8 w 75 n march 11th 18th 2001 is used in the study the results of the ice cover classification in the greenland sea are compared to model parameters from a sea ice model the classification of each ice pixel is performed using its backscatter and radiative properties as reflected in the polarisation ratio our results based on these comparisons show that the transformation into older mature sheet ice occurs within 5 10 days during one day the new ice cover increased by 33 000 km 2 the new ice appears in march 2001 as a peninsula maximum extent 56 000 km 2 appended to the belt of older ice drifting along the east coast of greenland these results are consistent with the ice model and with radarsat images furthermore using the ice model it is demonstrated that the new ice mature ice threshold in the classification corresponds to the physical transition of the ice cover from pancake ice to a consolidated young ice sheet the classification of each pixel into ice or water is done using two scatterometer parameters namely the polarisation ratio and the daily standard deviation of the backscatter

seismotectonics at the trench trench trench triple junction off central honshu we study earthquakes in and near the ttt type triple junction off boso peninsula central honshu to elucidate the plate interaction in this area the pacific north america northeast japan and philippine sea plates meet at the junction of the japan and izu bonin trenches and the sagami trough we determine focal mechanisms using wwssn data we also determine accurate focal depths by modeling body waves there is no serious trade off between focal depth and source time function for the events treated in this study

cryosphere applications of nscat data though designed to measure vector winds over the ocean new imaging techniques facilitate the use of nasa scatterometer data nscat in cryosphere studies nscat provides data of unprecedented coverage resolution and quality which when coupled with the scatterometer image reconstruction with filtering sirf algorithm enables images of spl sigma sup o at resolutions approaching 8 km over stationary targets such images are useful in ice mapping and classification and multidecadal studies are possible by comparison with seasat scatterometer sass data the utility of nscat data in polar ice studies is illustrated through a review of two cryosphere applications of nscat data 1 sea ice mapping and tracking and 2 ice sheet change in greenland and antarctica the wide swath and frequent revisit coupled with incidence and azimuth angle diversity makes nscat data very effective in mapping the extent of sea ice in greenland snow and ice facies are clearly delineated on the basis of the seasonally dependent radar backscattering cross section due to sensitivity of radar backscatter to diagenetic changes occurring at and beneath the surface comparison of nscat and sass data enables study of change in greenland between 1978 and 1996

temporal variation of large intraplate earthquakes in coupled subduction zones the focal mechanisms of intraplate earthquakes within subducting lithosphere are frequently used to infer large scale stress regimes induced by slab pull bending or unbending and lateral segmentation and undulations of the slab numerous studies have further postulated that the intraplate activity is influenced by transitory regional stress regimes such as those associated with interplate thrust events temporal variations of the latter type may potentially play an important role in assessing regions of uncertain seismic potential and possibly even in earthquake forecasting a systematic analysis of 1130 focal mechanisms for intraplate earthquakes with m b 5 0 in the depth range 0 300 km is conducted for nine circum pacific subduction zones all of which are known to have large interplate thrust events the spatial and temporal relationships of the earthquakes within the subducting slab to the large thrust events in each region are appraised the earthquake catalog assembled contains all published focal mechanisms and is probably complete for m b 6 5 for the years 1963 1986 for many of the localized regions considered in detail the catalog is complete to lower thresholds of m b 6 0 or m b 5 5 this analysis provides compelling evidence for a temporal link between large interplate thrust activity and intraplate seismicity for the seismically coupled regions considered here outer rise compressional events have occurred prior to several large thrust events or are associated with seismic gaps while outer rise tensional events generally only follow interplate ruptures in the intermediate depth range large down dip tensional events generally precede interplate thrusts and are often concentrated at the down dip edge of the coupled zone a transition to down dip compressional stress or diminished tensional activity at intermediate depth is observed after several large thrust events e g 1960 chile 1974 peru 1957 aleutian 1971 new britain these examples support the notion that the intraplate stress environment responds viscoelastically to the temporally varying interplate stress regime assuming that this concept is correct the seismic potential of several seismic gaps is considered on the basis of both outer rise and intermediate depth earthquake activity

double seismic zone in tonga the clear evidence of a double seismic zone beneath japan1 2 has led seismologists to search for other such zones elsewhere because they have been found in few other areas3 6 double seismic zones are thought to be rare and to depend strongly on a peculiar combination of external conditions such as age of the descending slab and the convergence rate 7 we report here a new double seismic zone which has been found beneath the tonga arc8 where the stress state in the subducting slab has been described as strongly down dip compressional7 9 10 this discovery suggests that the double seismic zone is a general feature of subducting slabs and may be found in many other subduction zones

complex distribution of large thrust and normal fault earthquakes in the chilean subduction zone summary we present the results of a systematic study of events with m 6 in northern chile 20 33 s for the period between 1963 and 1971 medium to large earthquakes near the coast of this region are of three types 1 interplate events at the interface between the downgoing slab and the overriding south american plate these events can be very large reaching magnitudes greater than 8 2 intra plate earthquakes 20 30 km inside the downgoing slab they have fault mechanisms indicating extension along the dip of the slab and may have magnitudes up to 7 5 3 less frequent m 6 events that occur near the top of the downgoing slab and have thrust mechanisms with an almost horizontal e w compressional axis this type of mechanism is very different from that of the events of type 1 which are due to shallow dipping reverse faulting there is a rotation of about 30 of the compressional axis in the vertical plane between events of types 1 and 3 three groups of events near 32 5 25 5 and 21 s were studied in detail depth and mechanisms were redetermined by p wave modelling and relative locations were obtained by a master event technique near 32 5 s only events of types 1 and 2 were found in the time period of this study at the two other sites the three types of events were identified this shows clearly that there are compressive stresses at the top of the slab and extension at the centre a situation which is usually found in the areas where a double benioff zone has been identified in the seismicity

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

large intermediate depth earthquakes and the subduction process this study provides an overview of intermediate depth earthquake phenomena placing emphasis on the larger tectonically significant events and exploring the relation of intermediate depth earthquakes to shallower seismicity especially we examine whether intermediate depth events reflect the state of interplate coupling at subduction zones and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes historic record of large intraplate earthquakes m b 7 0 in this century shows that the new hebrides and tonga subduction zones have the largest number of large intraplate events regions associated with bends in the subducted lithosphere also have many large events e g altiplano and new ireland we compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with m 6 and depth between 40 and 200 km the final catalog includes 335 events with 47 new focal mechanisms and is probably complete for earthquakes with m b 6 5 for events with m 6 5 nearly 48 of the events had no aftershocks and only 15 of the events had more than five aftershocks within one week of the mainshock events with more than ten aftershocks are located in regions associated with bends in the subducted slab focal mechanism solutions for intermediate depth earthquakes with m 6 8 can be grouped into four categories 1 normal fault events 44 and 2 reverse fault events 33 both with a strike nearly parallel to the trench axis 3 normal or reverse fault events with a strike significantly oblique to the trench axis 10 and 4 tear faulting events 13 the focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down dip extensional stress prevails in a gently dipping plate in contrast along decoupled subduction zones great normal fault earthquakes occur at shallow depths e g the 1977 sumbawa earthquake in indonesia type 2 events with strike subparallel to the subduction zone and most of them with a near vertical tension axis occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and consequently the change in focal mechanism from type 1 to type 2 events events of type 3 occur where the trench axis bends sharply causing horizontal parallel to the trench strike extensional or compressional intraplate stress type 4 are hinge faulting events for strongly coupled zones we observed temporal changes of intermediate depth earthquake activity associated with the occurrence of a large underthrusting event after the occurrence of a large underthrusting event the stress axis orientation of intermediate depth earthquakes changes from down dip tensional to down dip compressional e g 1960 chile 1974 peru 1982 tonga and 1952 kamchatka earthquakes or the number of large intermediate events decreases for a few years e g 1964 alaska and 1985 valparaiso earthquakes we conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate depth seismicity spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate depth events

effects of sedimentary structure and bathymetry near the source on teleseismic p waveforms from shallow subduction zone earthquakes summary the effects of sedimentary layers and non flat bathymetry on teleseismic p waveforms from shallow subduction zone earthquakes are examined by calculating synthetic seismograms the actual bathymetry and shallow sedimentary layers revealed by the seismic refraction studies are incorporated in 2 d velocity models of the oceanic trench region 2 d finite difference calculation is employed to get a complete response of the assumed non flat model structures the result of the modelling of wwssn long period seismograms shows that not only the non flat bathymetry but also thick sedimentary layers are essential to model the synthetic seismograms the lack of non flat bathymetry or sedimentary structure in the structural model often results in unrealistic synthetics in general the paths going across the trench axis are most affected this is caused by the steeply dipping ocean bottom and the thick sedimentary layers at the trench slope observed seismograms of the 1968 kurile thrust event mb 5 5 and the 1967 java normal event mb 6 0 show remarkable large amplitude later arrivals despite their moderate magnitudes a realistic model of the trench structure however reproduces remarkably well this character of later arrivals which shows that it is essential to incorporate the structure our results also indicate that we need precise knowledge on the trench structure to model the structural effects in detail on the gdsn long period seismograms the effect of the sedimentary layers is negligible because the rather narrow band response of the gdsn seismograph lies in a frequency range lower than the affected band however the effect of the non flat bathymetry is significant and can not be neglected

transition from interplate slip to double seismic zone along the kuril kamchatka arc during subduction the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth to characterize this transition we studied 44 large to moderate sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the kuril kamchatka arc precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances at depths of 30 50 km our results delineate a transitional thrust zone characterized by earthquakes showing thrust faulting on moderately dipping nodal planes the northwesterly dipping nodal plane dips approximately 40 some 10 15 steeper than the dip of low angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km for earthquakes along the transitional thrust zone secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds in the former case the transitional thrust zone is interpreted as the deep seated portion of the plate interface whose dip increases with depth the alternative implies that the transitional thrust zone occurrs below the plate interface within subducted crust of the pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km the inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region however a locked interface does not seem to affect the state of strain in the double seismic zone the upper compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km at this depth the two layers of seismicity in the double seismic zone are separated by as much as 30 40 km too large a distance to be accommodated by models of seismogenesis invoking either the gabbro eclogite transition or dehydration of subducted oceanic crust

earthquakes in the subducting slab beneath northern chile a double seismic zone abstract available mechanism solutions of most intermediate depth earthquakes in northern chile have t axes aligned subparallel to the direction of plate subduction down dip tension however the mechanism of the 1977 january 17 earthquake 24 9 s 68 7 w is quite different from this general pattern and shows an almost horizontal compression in east west direction the focal depth was reexamined by two independent methods master event method and travel time difference between direct and surface reflected waves and it was concluded that this earthquake occurred at about 120 km depth within the subducting slab the systematic discrepancies observed in the focal depths of intermediate earthquakes in northern chile between those obtained by the least squares method and those by travel time differences can be ascribed to the effect of a higher p velocity within the subducting plate there is a suggestion of a two layer like structure in the e w cross section at about 24 s if we only plot the earthquake foci determined by pp p time difference the 1977 earthquake either 1 1 is an event in the compressional part of the double seismic zone or 2 2 shows an anomalous stress orientation because it occurred near the break in the dip angle of the subducting slab

the production of ice and dense shelf water in the okhotsk sea polynyas this paper examines the ice and dense water production in the okhotsk sea coastal polynyas for the 1990 1995 winters the dominant polynyas occur on the northwest and northern shelves and in shelikhov bay we use an algorithm developed for the special sensor microwave imager ssm i to derive for each polynya the area and composition of thin ice and open water and a heat flux algorithm to derive the ice and brine production historic oceanographic observations show that the northwest shelf is the only north pacific region where the 26 8 potential density surface outcrops to the surface and is also that part of the okhotsk shelf where the densest water is observed to occur in support of these observations we find that the northwest shelf polynya is the dominant ice and brine producer contributing on average about 55 of the total production shelikhov bay is the second largest producer with about 25 of the total this region has been previously neglected by both oceanographic and remote sensing studies using a combination of two dense water production models we find that the 6 year average dense water production lies between 0 2 0 4 sv the ice and brine production for the dominant northwest shelf vary interannually by a factor of 3 while the production from all the northern polynyas varies by factor of 2 the source of the variability for the northwest shelf comes from the fact that the southwest to northeast trend of the coastline and the mean winter geostrophic wind velocities are roughly parallel which means that small variations in the wind direction yield large changes in the ice production

outer rise earthquakes and seismic coupling outer rise earthquakes can be separated into two basic categories based on focal mechanism compressional and tensional events reflecting compressional and tensional outer rise stress regimes we propose a model that connects the stress regime in the outer rise to the behavior of large subduction earthquakes and explains the focal mechanisms and both the temporal and spatial occurrence of outer rise earthquakes we find that compressional outer rise events tend to occur oceanward of seismic gaps in seismically coupled subduction zones or equivalently just prior to large subduction zone earthquakes the compressional stress regime is probably caused by slip in adjacent subduction zone segments there are two modes of occurrence for tensional outer rise earthquakes depending on whether they occur in coupled or uncoupled subduction zones in coupled subduction zones tensional events tend to occur after large subduction events as tensional stress from slab pull is temporarily transmitted to the outer rise in subduction zones that are inherently uncoupled tensional events can occur at any time therefore in uncoupled subduction zones the outer rise is always in tension and only tensional outer rise events occur there while in coupled subduction zones the outer rise is in a compressional state prior to large subduction earthquakes and a tensional state afterwards due to the large displacements

seismic detection of the untransformed basaltic oceanic crust subducting into the mantle summary subcrustal earthquakes occur in a slab like zone beneath sw japan in association with the subduction of the philippine sea plate they generate a distinct pair of p and s later phases with anomalously low apparent velocities of about 7 and 4 km s 1 which cannot be explained by a laterally homogeneous velocity structure these later phases are observed for particular source receiver geometries the sources at depths shallower than 50 60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust we examined the nature of these later phases using two methods one is to determine two types of apparent velocities an event common apparent velocity and a station common apparent velocity which may be different from one another for a laterally inhomogeneous velocity structure the other is to trace seismic rays numerically for a laterally heterogeneous velocity model an extensive analysis using these two methods leads to a conclusion that the slab like seismic zone constitutes a low velocity channel in the uppermost mantle with p and s velocities comparable to those of the lower continental crust beneath sw japan the waves trapped within the low velocity channel escape from it through its contact with the continental moho and are observed as a distinct pair of later phases the head waves guided by the bottom boundary of the low velocity channel are observed as weak arrivals of initial phases the thickness 10 km and the velocities vp 7 km s 1 vs 4 km s 1 of the subcrustal seismic zone indicates that it is made up of the untransformed basaltic oceanic crust subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50 60 km we present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in sw japan the models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental moho such features are qualitatively consistent with the results of explosion seismology and with the bouguer anomaly pattern in the relevant regions the oceanic lithosphere with the untransformed light oceanic crust at its top might still be buoyant after subduction a factor which must be taken into account in considering the driving mechanisms of plate tectonics the slab geometry and the earthquake related tectonic process

depletion of perennial sea ice in the east arctic ocean 1 the extent of perennial sea ice in the east arctic ocean 0 180 e decreased by nearly one half with an abrupt reduction of 0 95 106 km2 while the west arctic ocean 0 180 w had a slight gain of 0 23 106 km2 between 2004 and 2005 as observed by satellite scatterometer data during november december the net decrease in the total perennial ice extent is 0 72 106 km2 about the size of texas perennial ice in the east arctic ocean continued to be depleted with an areal reduction of 70 from october 2005 to april 2006 with the east arctic ocean dominated by seasonal sea ice a strong summer melt may open a vast ice free region with a possible record minimum ice extent largely confined to the west arctic ocean simultaneous scatterometer measurements of sea ice and winds will be crucial for sea ice monitoring and forecasts

downdip tensional earthquakes beneath the tonga arc a double seismic zone characteristics of seismicity at intermediate depth beneath the tonga arc are studied special attention is paid to newly identified downdip tensional events because the dominance of downdip compression has been known in this area we reexamine the relatively shallow 70 km downdip tensional events reported in the harvard centroid moment tensor solutions by modeling bodywave waveforms of world wide standard seismograph network seismograms and conclude that they are in fact intraplate downdip tensional events furthermore these downdip tensional events are consistently located below and seaward of downdip compressional events and appear to constitute a double seismic zone the result of a relative relocation technique applied to these earthquakes also supports this observation which was originally based on routinely determined epicenters the subducting slab beneath the tonga arc has been considered as a prototype of a compressionally loaded slab the existence of a double seismic zone in tonga therefore would suggest that double seismic zones located at intermediate depth in subducting slabs are more common than previously thought

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

thrust and extensional faulting under the chilean coast 1965 1971 aconcagua earthquakes summary earthquakes under coastal northern chile from 18 s to 33 s occur at two very different depths the shallower interplate events have the typical thrust mechanisms associated with subduction of the nazca plate under south america the deeper close to 70 km events have normal fault mechanisms with a tension axis approximately parallel to the dip of the downgoing slab we study in detail a couple of ms 7 5 events that took place about 60 km north of valparaiso chile the 1971 july 9 event was a shallow thrust interplate event at 40 km depth while the 1965 march 28 earthquake had a normal fault mechanism and a depth of 72 km we carry out a detailed analysis of long period p and surface waves observed in the wwssn network of stations in order to improve fault plane solutions and depth determinations for the 1971 event we find a seismic moment of 5 6 1027 dyne cm from both p and surface waves the source area for this event is relatively small compared to that of other underthrust events with similar moments assuming a circular fault shape and a rigidity of 7 1011 dyne cm 2 we find a stress drop of 38 bar the 1965 event is more complex seismic moments of 1 8 and 1 0 1027 dyne cm are found from surface and body waves respectively the stress drop estimated from p waves and a circular fault model is close to 91 bar the existence of earthquakes at two different depths near the chilean coast may be interpreted in terms of a double layered benioff zone as has been found in honshu kuriles or central peru unfortunately the hypo central determinations of events in this region are not precise enough to separate the two layers an alternative method to demonstrate the existence of a double benioff zone is to look for compressional events associated with the upper layer of the down going slab a search among available fault plane solutions is ambiguous three shallow coastal events are found in stauder s catalogue with horizontal pressure axes normal to the trench but they might be intra continental plate events

how does the amur river discharge flow over the northwestern continental shelf in the sea of okhotsk abstract the paths of the amur river discharge on the continental shelf in the sea of okhotsk are still unknown despite their significance in transporting dissolved and particulate iron in this study we conduct a coupled ice ocean simulation for the northern sea of okhotsk from june 1998 to september 2000 to answer the question does the amur river discharge deposit materials to the pathway of the dense shelf water in a series of numerical experiments we identified two routes the western and eastern routes that could transport the river water more than 100 km offshore over the northwestern continental shelf the two routes share the clockwise gyre in the sakhalin gulf and the northeastward flow on the northwestern continental shelf these features are connected through the westward jet along the slope from the sakhalin gulf the western route and the northward transport over the shelf break canyon the eastern route the river water the dense shelf water and the easterly wind are in a fine geophysical balance for those features and all are required for the formation of the two routes the model results show that these unique joint effects in the sea of okhotsk allow the amur river discharge to be effectively transported over the northwestern continental shelf unlike a general river discharge that flows along the coast and deposit materials into the pathway of the dense shelf water

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

depth distribution of moment release in underthrusting earthquakes at subduction zones we present a global analysis of the depth distribution of moment release along the thrust contact at subduction zones using data from harvard s centroid moment tensor catalogue substantial regional variations in the depth distribution of moment release are observed a significant correlation exists between the depth of maximum moment release and the convergence rate of subducting lithosphere and the sediment thickness at the trench subduction zones with fast convergence rates and thick sediment accumulations concentrate moment release at greater depths than subduction zones with slow convergence rates and little sediment accumulation the correlation between the convergence rate of oceanic lithosphere and the depth distribution of moment release may be best explained by differences in the thermal structure of the plate interface subduction zones with fast convergence velocities transport cold material to greater depths than subduction zones with slow convergence velocities the colder temperature of the deep portions of the thrust contact at subduction zones with fast convergence velocities allows seismic failure to occur at greater depth than possible in subduction zones with slow convergence velocities as has been demonstrated in intraplate environments our observations in interplate thrust settings support the idea that temperature is the most important factor controlling the maximum depth of seismic failure in subduction zones with thick sediment accumulations the stable sliding properties of unconsolidated sediments may be responsible for the low levels of shallow moment release

sea ice identification using dual polarized ku band scatterometer data this paper describes a classification algorithm using dual polarized scatterometer measurements to identify the edge of the sea ice cover the distinct polarization scattering signatures of sea ice and open water are discussed and illustrated with the dual polarized radar measurements from the seasat a scatterometer sass the analysis of sass data suggests that the ratio of vertical and horizontal polarization backscatter denoted as the copol ratio is a useful discriminator of sea ice and open ocean a simple classification algorithm using the thresholds of the copol ratio and backscatter levels is proposed the feasibility of this algorithm is demonstrated using the sass data from the single sided dual polarization mode the results indicate that the dual polarized measurements from the nasa scatterometer nscat can be used to produce routine maps of sea ice edges

investigations of newly formed sea ice in the cape bathurst polynya 2 microwave emission 1 this study examines the role of newly formed sea ice geophysical state on microwave emission coincident with sea ice geophysical sampling ship based passive microwave emission data dual polarized at 19 37 and 85 ghz were collected in the cape bathurst polynya during 18 october and 13 november 2003 using polarization ratios prs we found that bare thin ice was separable from snow covered ice thin snow equal to 0 02 0 13 m thickness is significantly correlated with the spectral gradient ratios grv 85 19 r2 0 55 p value 0 2 the relationship between atmospherically corrected r37 and bare ice thickness showed an exponential relationship very comparable to that reported by martin et al 2004 which is ascribed to the reduction of bare ice surface salinity based on both observational and modeling studies however the relationship quickly becomes invalid for even thin snow covered ice due to significant impact of thin wet liquid water fraction 0 02 0 04 snow on microwave emission our results suggest that the sea ice algorithms nasa team and nasa team 2 could underestimate total ice concentration over thin bare ice by 35 on average while both algorithms underestimate the total ice concentration by 20 over snow covered ice using pr 85 sea ice could be delineated from open water using a properly adjusted threshold value accounting for cloud or fog effects possibly with the exception of dark nilas and or bare consolidated pancakes

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

an intermediate depth earthquake beneath tibet source characteristics of the event of september 14 1976 this paper presents the results of a detailed study of an intermediate depth earthquake beneath southern tibet september 14 1976 0643 51 6 ut 29 8l n 89 57 e focal depth 90 10 km mb 5 4 from the differences in arrival times of p pp and sp and from rayleigh wave spectral densities the focus is estimated to be at depth of 90 10 km and therefore in the uppermost mantle from the polarities of the first motions of p waves inversion of the relative amplitudes of p pp and sp and synthesis of their waveforms the source mechanism is constrained to be predominantly normal faulting with the t axis approximately horizontal and trending east west this mechanism is similar to those of shallow earthquakes in tibet the occurrence of an earthquake at depth of 90 km is consistent with rapid deformation at a temperature of about 700 to 800 c in the mantle beneath the tibetan crust the seismic moment of an equivalent double couple point source is estimated to be about 9 1024 dyne cm with an uncertainty factor of 1 3 from the sp amplitudes in the period range near 10 s and 5 1024 dyne cm also with an uncertainty factor of 1 3 from the spectral density of rayleigh waves with periods of 50 s

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

double seismic zone beneath the mariana island arc a double zone of seismicity has been found beneath the northern marianas in the depth range 80 120 km the benioff zone undergoes a pronounced bend with radius of curvature about 200 km before straightening and descending nearly vertically to a depth of more than 600 km thus this is an ideal area in which to search for evidence of stresses associated with unbending of the subducted oceanic plate we relocated all teleseismic events from 1963 through 1975 by using a regional composite master event technique which significantly decreased the scatter of the hypocenters the three events of the lower seismic zone are separated from the upper zone by about 30 35 km the true thickness of the upper seismic zone is probably 20 km or less the limited focal mechanism data indicate that there is downdip tension in the lower zone and downdip compression in the upper zone the seismicity patterns could be generated either by thermal stresses or by unbending a plate which was originally deformed anelastically we show that an elastic perfectly plastic model of the rheology of the plate can account for the separation of the two seismic zones the focal mechanisms and the greater activity of the upper seismic zone

thickness and production of sea ice in the okhotsk sea coastal polynyas from amsr e 1 from comparisons with thickness of sea ice from advanced very high resolution radiometer avhrr and ice profiling sonar data we have developed an advanced microwave scanning radiometer eos amsr e thin ice thickness algorithm for the sea of okhotsk this algorithm can estimate ice thickness of 0 2 m without snow using the polarization ratio of amsr e brightness temperature at a 36 5 ghz channel from a linear relationship with avhrr ice thickness when a snow cover exists on the thin ice surface as occurred a few times in each winter it is shown that the algorithm cannot detect the thin ice sea ice and dense shelf water dsw production in coastal polynya are estimated on the basis of heat flux calculation with the daily amsr e ice thickness for three winters december march of 2002 2003 to 2004 2005 the ice production is largest in the northwest shelf nws polynya which accounts for 45 of the sum of ice production in major coastal polynyas the ice production in major coastal polynyas would cover the maximum ice area of the okhotsk sea if the average ice thickness is assumed to be 1 m variability of the ice production is mainly modulated by air temperature in the nws polynya which is the main dsw production area the annual dsw formation rate is estimated to be 0 36 sv

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

earthquake generating stresses in a descending slab abstract a numerical study of stress within descending slabs beneath island arcs provides a general interpretation for the occurrence of deep and intermediate focus earthquakes the computation of stress due to several factors which have been proposed for the interpretation of the double planed seismicity of intermediate depth events shows that thermal expansion and olivine spinel phase change play an important role in earthquake generation the stress field due to the combination of these two causes gives the following results in the depth range 50 300 km the compressional and tensional stress predominates in the upper and central part of the slab respectively maximum shear stress amounting to 1 6 gpa in compressional and 0 4 gpa in tensional fields occurs at a depth of 200 km the stress is generally very small at around 300 350 km below 400 km compressional stress exists at the central part of the slab and tensional stress appears near the upper and lower surfaces of the slab maximum shear stress is 2 8 gpa in compression and 1 6 gpa in tension at a depth of 500 km the principal axes in these high stress fields align almost parallel to the dip direction of the slab these stress features explain well such observed seismological facts beneath island arcs as the seismicity depth relationship planer configuration of seismic zone existence or non existence of double planed seismicity and focal mechanisms of intermediate and deep focus earthquakes

spherical shell tectonics effects of sphericity and inextensibility on the geometry of the descending lithosphere the shape of a deep seismic zone is thought to represent that of the descending slab of lithosphere the lithosphere before subduction is a spherical shell and the shape of the descending slab is the result of the deformation of the spherical lithosphere at the subduction zone upon bending a spherical shell often deforms in a very different way from a simple plate we examine whether the actual shape of the descending slab can be explained by a simple bending of an inextensible spherical shell which shows little surface deformation under moderate stress this examination is made region by region for most of the subduction zones in the world by means of an analogue method the lithosphere is simulated by an inextensible spherical shell made of polyvinyl chloride resin the wadati benioff zone is shaped by plaster by referring to the reliable hypocentral data selected from the international seismological centre isc bulletins the spherical shell is forced to fit the miniature of the wadati benioff zone fitting is first attempted only by bending if a good fit is not attainable and if a discontinuity or gap in seismic activity is observed in the relevant region the spherical shell is torn along this discontinuity or gap and the goodness of fit is reexamined the results of the analysis are summarized as follows 1 the shape of the wadati benioff zone can be simulated largely by a simple bending of a spherical shell without surface deformation 2 in almost all of the regions of poor fit with bending a good fit can be achieved by tearing the spherical shell along the trace of low seismicity the sphericity of the lithosphere and the inextensibility upon deformation are the two essential factors in controlling the slab shape this means that the lateral constraint is most important for understanding the geometry of the downgoing slab of lithosphere and the stress state within it further several problems related to the deformational characteristics of the spherical lithosphere are also reviewed and discussed in connection with subduction tectonics

role of shallow phase changes in the subduction of oceanic crust detailed studies of the seismicity of several subduction zones demonstrate that shallow dipping thrust zones turn to steeper angles at depths of about 40 kilometers an increased downward body force resulting from shallow phase changes in subducted oceanic crust may be the cause of this increased dip angle in addition the volume reduction associated with phase changes may produce sufficiently large stresses in neighboring rocks to cause the seismicity of the upper benioff zone

thermal stresses in subducting lithosphere can explain double seismic zones the discovery of double planed zones of intermediate depth seismicity associated with subducting oceanic lithosphere refs 1 7 and l s h and k h j in preparation has generated much interest in the origin and distribution of stresses in descending plates several mechanisms have been proposed to explain why double zones occur8 here we investigate the role of thermal stresses for the state of stress in a descending plate and test whether they can account for the observed characteristics of double seismic zones by modelling the descending slab as a thin elastic plate and using the temperatures in the slab estimated by toksoz et al 9 we find that the polarities of thermally generated stresses agree with those obtained from the focal mechanisms of earthquakes in the two zones thermal stresses also can explain the separation between the two zones of seismicity at shallow depths and their gradual merging at greater depth

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

double seismic zone beneath the middle of hokkaido japan in the southwestern side of the kurile arc abstract the vertical section of microearthquakes determined accurately by using the hokkaido university network shows two dipping zones the double seismic zone 25 30 km apart in the depth range of 80 150 km beneath the middle of hokkaido in the southwestern side of the kurile arc hypocentral distribution of large earthquakes m b 4 based on the isc international seismological centre bulletin also shows the double seismic zone beneath the same region the hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone the difference in seismic activity between the two zones beneath hokkaido is in contrast with the region beneath northeastern honshu in the northeastern japan arc composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down dip extension for the lower zone as is observed beneath northeastern honshu for the upper zone however the stress field is rather complex and not necessarily similar to that beneath northeastern honshu this may be considered to indicate the influence of slab contortion or transformation in the hokkaido corner between the kurile and the northeastern japan arcs

existence of an s wave reflector near the upper plane of the double seismic zone beneath the southern kanto district japan the amplitude of coda from local shallow earthquakes in the southern kanto district of japan does not decay smoothly with time a clear phase appears in the s coda at lapse time from 15 to 45 s measured from the s wave arrival and it is predominant for frequencies around 8 hz this phase seems to be an s wave reflected from a discontinuity inclined westward at upper mantle depths since it is predominantly on the horizontal components and propagated westward with a high apparent velocity applying an inversion method to the travel times of the phases read from 8 hz band seismograms we locate the s wave reflector near the upper plane of the double seismic zone associated with the subducting pacific plate the s wave reflector is resolved east of the volcanic front at a depth of 80 100 km if we put qs l 10 3 the reflection coefficient is estimated to be around 0 5 however the scatter is large the s wave impedance contrast at the interface seems to be very large it may suggest the existence of a zone containing many liquid bodies

effects of near source seafloor topography on long period teleseismic p waveforms in the study of the shallow earthquakes in the subduction zones reverberations inside the water layer near the source are known to affect very significantly the far field body waveforms the usual correction is to take a horizontally layered model as the near source structure but it is not realistic for the trench geometry therefore finite difference simulation combined with far field ray theory is employed in order to examine the effects of the realistic non flat seafloor topography as a realistic trench model the japan trench is selected the response of the structure near the source is calculated by two dimensional finite difference simulation then teleseismic p waveforms are calculated by using reciprocal theorem our results suggest that 1 around the trench bottom effects of non planar structure are remarkable and the realistic seafloor cannot be approximated by any model of planar structure however 2 effects of the continental slope are well approximated by a model of planar dipping seafloor of appropriate dip therefore the near source structure must include the realistic seafloor topography or the waveform analyses may lead to spurious subevents in a later stage of rupture process and mislocation of events our results also show that present knowledge about the seafloor topography is enough to incorporate the complicated geometry

lithosphere ionosphere coupling mechanism and its application to the earthquake in iran on june 20 1990 a review of ionospheric measurements and basic assumptions abstract analysis of the reliable ionospheric data that have been obtained by different research groups near a seismically active region in western iran has enabled us to infer that 1 disturbances in the lower ionosphere h 90 km radio wave phase anomalies f region h 270 km airglow anomaly and upper ionosphere h 2400 km vlf noise anomalies measured by the satellite were recorded at distances of 250 km 1000 km and 2000 km respectively from the epicentral zone these might be considered as precursors h days before the event and or as after effects of seismical activity 2 disturbances in the lower ionosphere correspond to density enhancement in this part of the ionosphere 3 decreasing the 630 nm emission intensity in the f region may be interpreted as a formation of the region with plasma depletion 4 anomalous impulse vlf emissions in the upper ionosphere apparently may be related with whistler trapping into the ducts formed in the ionosphere above the seismically active region 5 lh noise frequency rising corresponds to increasing the plasma density in the upper ionosphere to interpret these anomalies we exploit a new lithosphere ionosphere coupling mechanism related to internal gravity wave generation in seismically active regions under favourable conditions these waves can reach the ionosphere the quantitative analysis of ionospheric disturbances caused by internal gravity wave interaction with ionospheric plasma will be published elsewhere

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

effects of near source bathymetry on teleseismic p waveforms teleseismic p waveforms from submarine earthquakes show water reverberations that are often poorly modeled by synthetic seismograms this study suggests that considerable misfit results from the non horizontal nature of the seafloor in the epicentral region and examines this effect for a planar dipping water crust interface geometric ray theory synthetics calculated for a double couple source show that p waveforms are extremely sensitive to seafloor orientation because of the large impedance contrast across the boundary seafloor dips as small as 0 5 significantly modify the ray paths affecting the take off angle at the source reflection and transmission coefficients at the interface geometric spreading and travel time to test the applicability of such modeling p waveforms from the march 29 1976 cocos ridge intraplate earthquake were studied the waveforms many of which show prominent water multiples poorly modeled with flat lying bathymetry were inverted for the best fitting planar ocean bottom orientation using a grid search technique synthetics computed for the best fitting seafloor orientation match the data much better than synthetics for a flat lying structure and reproduce the observed azimuthal dependence of water multiple amplitudes the derived ocean bottom dip angle and direction are in agreement with bathymetric charts suggesting p waveforms provide constraints on the seafloor orientation in the epicentral region green s functions incorporating the effects of bathymetry should provide better resolution of the source parameters of submarine earthquakes

bathymetric effects on body waveforms from shallow subduction zone earthquakes and application to seismic processes in the kurile trench resolution of earthquake source characteristics is limited by structural complexities in the source region this study explores the effects of dipping near source bathymetry and structure on long period teleseismic body waveforms from shallow subduction zone earthquakes ray theory synthetics calculated for shallow double couple sources and velocity structures incorporating planar dipping interfaces suggest that p waveforms are most sensitive to water depth and sea bottom orientation for sources beneath a substantial water layer the seafloor dip of the inner trench slope can give rise to prominent later arrivals and waveform complexity not found on synthetics calculated for horizontal bathymetry sediment thickness and the dip of the sediment crust and crust mantle interfaces generally have lesser effects the high amplitude arrivals result largely from changes in the source takeoff angles of the reverberations these arrivals present difficulties for source depth and time function determination from p waveforms and may cause spurious complexity in the derived pattern of earthquake moment release sh waveforms show little effect of sea bottom orientation or local structure for shallowly dipping thrust faulting events suggesting that sh waveform analysis can stabilize source parameter studies with respect to unknown or incompletely parameterized bathymetry and structure effects of nonhorizontal structure are observed in the p waveforms from shallow kurile trench events ms 5 8 7 2 which show prominent later arrivals not observed on sh waveforms synthetics calculated for a dipping water crust interface provide a better fit to these arrivals than those calculated for a horizontal interface suggesting that the p waveforms show systematic effects of seafloor dip the synthetics are highly sensitive to the assumed orientation of the seafloor the data are fit best for seafloor orientations consistent with the dip of the inner trench slope the distorting effect of the water layer gives rise to systematic differences in p wave complexity between shallow events near the seismic front which occur beneath a thick water layer and show numerous later arrivals and deeper arcward events which occur beneath a thin water layer and show simple waveforms large reverberations in the p waveforms of the october 20 1963 and june 10 1975 kurile tsunami earthquakes have previously been used as evidence for a long source duration and a change in focal mechanism during rupture however sh waveforms suggest simple source time functions with durations of 25 and 43 s inversion of p and sh waveforms and amplitudes yields shallowly dipping thrust mechanisms with no conclusive evidence for a previously proposed steepening of the fault dip during rupture the large p wave reverberations cannot be fit using a point source and a horizontal seafloor detailed modeling of the 1963 event as an upward propagating rupture extending through low velocity sediments to the ocean bottom combined with the effects of the dipping seafloor allow the reverberation amplitudes to be matched the anomalous tsunami excitation appears to be related to the location of the sources at shallow depths within low velocity sediments

stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle an elementary mechanical model of a coupled subduction zone is presented it is used to study the transfer of extensional stresses along the underthrusting plate and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate during the earthquake cycle the model assumes an elastic plate which deforms one dimensionally has shear drag interactions treated approximately by the elsasser procedure with a maxwellian viscoelastic mantle and is subject to steady gravitational sinking forces great thrust earthquakes are simulated as sudden stress relieving displacements at the contact zone between the slab and overriding plate the model allows analysis of the space and time variable stresses in the main thrust zone the adjacent oceanic plate and the subducting slab over each earthquake cycle stress variations leading to outer rise events are discussed as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone based on simulations of periodic great earthquakes the results of simulated slow progression of deformation slip from below into the thrust contact zone are also presented such preslip occurring over a time scale that is small in comparison with the whole seismic cycle e g of the order of a tenth or so is shown to transiently slow the rate of buildup of or even relieve extensional stresses in the slab in the absence of preslip those stresses build up steadily in time during the latter part of the cycle the results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world the observations include tensional outer rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle normal tensional intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab

mapping of sea ice production for antarctic coastal polynyas 1 active sea ice production in antarctic coastal polynyas causes dense water formation finally leading to antarctic bottom water aabw formation this study gives the first mapping of sea ice production in the antarctic ocean based on heat flux calculation with ice thickness data derived from satellite data the highest ice production occurs in the ross ice shelf polynya region the ice production there decreased by 30 from the 1990s to the 2000s which can be one candidate for causing the recent freshening of aabw the cape darnley polynya in east antarctica is found to be the second highest production area suggesting a possible aabw formation area according to our estimation around 10 of southern ocean sea ice is produced in the major antarctic coastal polynyas the mapping provides surface heat and salt flux conditions in the ice covered region which have not been well understood

earthquakes plate subduction and stress reversals in the eastern aleutian arc plate subduction beneath the 1500 km long segment of the eastern aleutian arc between kodiak and atka islands 154 w and 176 w longitude is studied with observations from teleseismic data the primary data base consists of hypocenters of earthquakes for the period 1965 1975 carefully selected from the bulletins of the international seismological centre and of 44 new focal mechanism solutions the principal results of this study are that hypocenters of intermediate depth earthquakes in the eastern aleutians appear to define a weakly developed double seismic zone at depths between 70 and 170 km additional evidence for a double seismic zone comes from focal mechanisms which generally show downdip directed p axes for earthquakes in the upper zone and downdip directed t axes in the lower zone major features of the double zone can be explained by thermoelastic stresses in the downgoing plate the observed predominant downdip stress polarity at intermediate depths in the descending plate reverses along strike of the arc this stress reversal coincides in map view with a change from a continental to an oceanic arc the coincidence may result from spatial differences either in the coupling between the plates at shallow depths or in the rheology of the surrounding oceanic versus continental mantle alternatively the stress reversal may be related to the time since the last great earthquake if the observed stress polarity relates to the elapsed time since the last great earthquake it may be a significant observation for long to intermediate term prediction of great thrust earthquakes in subduction zones portions of the eastern aleutian arc where downdip tension predominates contain one or more seismic gaps that appear to have a high probability for great earthquakes in the next few decades a moderate sized bathymetric feature in the pacific plate the amlia fracture zone modifies subduction and produces a bend or offset in the downgoing plate as well as an offset in the line of active volcanoes in the upper plate subduction of the fracture zone appears also to cause formation of summit basins on the arc platform and some aseismic behavior of a 100 km long segment of the main thrust zone

triple seismic zone and the regional variation of seismicity along the northern honshu arc the regional variation of seismicity along the northern honshu arc japan is studied using accurate focal depths and focal mechanism types we use focal depths determined from pp p time intervals reported in the isc bulletins for submarine earthquakes depths are corrected by considering the pp phase reported in the bulletins as the pwp phase the reflection from the ocean surface out of more than 600 well located earthquakes selected from the isc bulletins we determine the types of the focal mechanisms of 184 events using p wave first motion data based on historical seismicity of great and large earthquakes we divide the zone of thrust type earthquakes at the plate interface into two regions the shallow thrust zone 0 40kin where great earthquakes ms 8 0 occur and the deep thrust zone 40 60km where large earthquakes ms 7 4 occur the activity of great or large earthquakes shows a variation along the are in some regions both the shallow and deep thrust zones are active and in other regions only one of the thrust zones is active the seismicity of recent moderate size earthquakes mb 4 combined with the focal mechanism type shows a variation along the are which reflects the variation of the activity of great or large earthquakes where large earthquakes do not occur in the deep thrust zone neither thrust type nor down dip compression tension type events occur in and beneath the deep thrust zone where large earthquakes do occur in the deep thrust zone we find a number of thrust type earthquakes further in the latter case in some regions the down dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple planed structure of seismicity the triple seismic zone this study confirms the hypothesis of previous workers seno and pongsawat 1981 on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone i e the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling respectively here the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone triple seismic zones are found offshore of miyagi prefecture where the deep thrust zone has been broken recently in 1978 and offshore of fukushima prefecture we expect a future large earthquake at the deep thrust zone offshore of fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was ruptured in 1938

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

seismic coupling and outer rise earthquakes variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events this variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone outer rise seismicity is strongly correlated with variations in interplate coupling reflecting the stress state of the interplate coupled zone over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling these events occur within the downgoing i e oceanic plate near the bathymetric trench axes and generally fall into the categories of tensional normal or compressional thrust with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench in uncoupled subduction zones only tensional outer rise earthquakes occur which indicates that the outer rise is dominated by tensional stresses associated with plate bending and or slab pull forces in strongly coupled subduction zones both tensional and compressional outer rise events are found these events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle in the strongly coupled regions tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone in four instances compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2 4 7 and 19 years after the associated outer rise event the remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown the occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region thirty compressional outer rise events have been located in trench segments of middle and south america the kurile islands the tonga and kermadec islands the new hebrides arc and the solomon islands regions in both the southern kamchatka and northern new hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional following a previous large underthrusting event to compressional at present thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present requiring only the occurrence of the next underthrusting event to complete the cycle the occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale

the double seismic zone in kuril kamchatka the tale of two overlapping single zones the classic double seismic zone in the central portion of the kuril kamchatka arc seems to turn into a regular wadati benioff zone with downdip compression and extension in the northeastern and southwestern ends of the arc respectively to delineate this lateral changeover we studied 33 large to moderate sized mb 5 1 earthquakes that occurred at depths shallower than 200 km between 1963 and 1991 these events took place in two regions where the double seismic zone terminates and single seismic zones emerge precise earthquake source parameters including focal depths and mechanisms are determined from a careful analysis of a mixture of analog and digital p and sh waveforms recorded at teleseismic distances our results indicate that the double seismic zone is confined between approximately 46 and 53 n northeast of 53 n intermediate depth earthquakes 70 200 km show only downdip compressional focal mechanisms furthermore the wadati benioff zone there seems to be a continuation of the upper compressional layer of the double seismic zone thus the changeover from a double to a single seismic zone in this case is simply disappearing of the lower extensional sheet of the double seismic zone similarly southwest of 46 n intermediate depth earthquakes show only downdip extensional mechanisms the single seismic zone is unlikely to consist of two layers of extensional seismicity in fact nowhere along the entire arc did we observe two distinct seismogenic layers with a single state of strain in other words these extensional events did not occur in the same seismogenic structure as the upper compressional layer of the double seismic zone however we cannot ascertain whether the single seismic zone in extension is simply a southwestern extension of the lower extensional layer of the double seismic zone lateral variation from a single zone under extension in the southwest to a double seismic zone and finally to a single zone under compression cannot be accounted for by conventional models of a double seismic zone on the other hand the addition of a component of stress transmitted from depths greater than 250 km can explain the observed lateral variation in the state of strain at intermediate depths because the slab between depths of 250 450 km is under extension in the southwestern end of the arc whereas the rest of the arc is under compression vanishing of the lower extensional layer of the double seismic zone toward the northeastern end of the arc is interpreted as a result of compression transmitted from below overshadowing extension caused by effects such as unbending a similar reasoning applies to the southwestern termination of the double seismic zone

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

influence of fault plane heterogeneity on the seismic behavior in the southern kurile islands arc the seismic behavior in the southern kurile islands arc is strongly influenced by spatial heterogeneity in the mechanical properties of the plate interface maps showing sites of maximum seismic moment release for the great underthrusting earthquakes of 1958 1963 1969 and 1973 provide a first order image of the major regions of enhanced strength or asperities hypocenters of these and an additional 83 smaller earthquakes with mb 5 3 and known underthrusting mechanisms were recomputed using the method of joint hypocenter determination in order to assess better the influence that the strength distribution exerts on the interplate seismicity patterns nearly all of the smaller magnitude earthquakes locate outside of the principal asperities of the great earthquakes this observation is consistent with substantial release of accumulated strain energy in asperity regions during the great earthquakes and a redistribution of stress in adjacent areas following their rupture an along dip depth dependent component of strength heterogeneity appears to influence the seismicity patterns the asperities of the 1958 and 1973 great earthquakes and hypocenters of most of the ms 7 earthquakes occur near the downdip edge of the 100 km wide seismically active plate interface this would be expected if shear resistance on the interface increases monotonically with lithostatic pressure up to the brittle ductile or stick slip stable slip transition that defines the base of the seismogenic zone the asperities of the 1963 and 1969 earthquakes however appear to be located well above the downdip edge of the coupled plate interface the occurrence of large asperities in middepth ranges of the seismogenic interface suggests the influence of along dip changes in other parameters besides lithostatic pressure that control shear resistance along strike segmentation of the kurile islands thrust zone may be a consequence of along strike alternation in the location of the largest asperities between the deep interface and the midinterface

regional variations of amplitudes of scsp phases observed in the japanese islands summary scsp phases are examined with special reference to their amplitude variations over seismograph networks in the japanese islands scsp of rela tively large amplitude are observed along the pacific side of the japanese islands amplitude ratios of the scsp with respect to scs are explained in terms of scs to p conversion at the upper boundary of a descending pacific plate which dips at about 30 and whose seismic wave velocity is between 5 and 10 per cent higher than that of the surrounding mantle large amplitude scsp are observed at stations nearest the kurile or japan trench the ampli tude ratios of scsp scs are found to decrease rapidly with distance inland from the trench an estimate is made of the effects of sharpness of the con version interface the incident angle of scs upon it and lateral variations of anelasticity within the asthenosphere overlying the descending pacific plate upon the scsp scs ratios we show that the sharpness of the conversion inter face is the dominant factor this result implies that the nature of the scs to p conversion interface varies with distance from the trench or equivalently with depth the presence of a low velocity zone in the vicinity of the conversion interface is inferred from the relative polarity between scsp and scs observed at the stations on the outermost side of north eastern japan scsp arrivals from shallower conversions at depths less than about 80 km observed in central and south western japan are also examined to elucidate the structure of the philippine sea plate descending beneath this region a discussion is made on the correlations between dipping subcrustal seismicity and the scs to p conversion interfaces except for the chugoku region the scs to p conversion interfaces correlate well with the dipping subcrustal seismicity our scsp observations indicate the presence of a dipping dis continuity in the uppermost mantle beneath the chugoku region where no subcrustal earthquakes have been detected

interplate coupling and temporal variation of mechanisms of intermediate depth earthquakes in chile we investigated the temporal variation of the mechanism of large intraplate earthquakes at intermediate depths in relation to the occurrence of large under thrusting earthquakes in chile focal mechanisms were determined for three large events 1 march 1934 m 7 1 d 120 km 20 april 1949 m 7 3 d 70 km and 8 may 1971 m w 7 2 d 150 km which occurred down dip of the great 1960 chilean earthquake m w 9 5 rupture zone the 1971 event is down dip compressional strike 12 dip 80 and rake 100 the 1949 earthquake focal mechanisms is 350 70 and 130 the data available for the 1934 event are consistent with a down dip tensional mechanism thus the two events which occurred prior to the great 1960 chilean earthquake are down dip tensional published fault plane solutions of large intermediate depth earthquakes 28 march 1965 and 7 november 1981 which occurred down dip of the valparaiso earthquakes of 1971 m w 7 8 and 1985 m w 8 0 are also down dip tensional these results suggest that before a major thrust earthquake the interplate boundary is strongly coupled and the subducted slab is under tension at intermediate depths after the occurrence of an interplate thrust event the displacement on the thrust boundary induces transient compressional stress at intermediate depth in the down going slab this interpretation is consistent with the hypothesis that temporal variations of focal mechanisms of outer rise events are due to changes of interplate coupling

seismic coupling along the chilean subduction zone underthrusting at subduction zones can cause large earthquakes at shallow depths but is accommodated by aseismic creep below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding we have determined the maximum depth of the coupled zone and its variability along the chilean subduction zone the maximum depth of seismic coupling is defined by the depth of large m 6 underthrusting earthquakes that have occurred at the downdip edge of the coupled plate interface earthquake depth is determined with omnilinear waveform inversion of long period p waves the statistical uncertainty in the depth is estimated using bootstrapping omnilinear inversion formally accounts for the scaling incompatibility between the p waveforms and decreases the uncertainty in the depth estimate we have found the depths of 27 earthquakes in the time period from 1961 to 1987 seismic coupling in chile extends down to 48 53 km there is a resolvable change in the maximum depth of coupling around 28 s the region immediately north of this latitude has a coupled zone that extends no deeper than 36 41 km while south of this latitude coupling extends down to 48 53 km this transition is not simply related to the oceanic lithospheric age and plate convergence rate but coincides with a change in several geophysical and geological phenomena with respect to shallower north and deeper south coupling there is a change from active volcanos to no volcanos the dip of the deep slab changes from steep to shallow the thickness of trench sediments changes from thin to thick and the depth of the oceanic basement changes from deeper to shallower at 28 s due to the multiplicity of changes at 28 s there is no clear unique interpretation of what physical mechanism controls the variability in the maximum depth of the seismically coupled zone in chile