seismic and volcanic activities and aseismic movements as plate motion components in the aegean area abstract slip rates varying between 0 22 and 2 18 cm yr 1 are determined from seismicity in several segments of the aegean fore arc and back arc regions the rate along the main zone of plate interaction is about 0 5 cm yr 1 while aseismic slip rate equal to 2 5 or 0 5 cm yr 1 depending on the plate motion rate assumed indicates that at least 50 of the plate motion is accommodated by creep these values with respect to the rate of volcanic eruption 0 13 eruptions per year 1000 km of arc length reinforce the suggestion that active volcanism in subduction zones is related to aseismic subduction the eruption rate is however anomalously low with respect to the degree of plate decoupling 0 83 or 0 5 implying that volcanism is not necessarily related to the degree of decoupling and suggesting that the high aseismic slip component of the plate motion is taken up by subcrustal seismicity a tentative hypothesis is made that the hellenic subduction zone is of intermediate type between the marianas and chile subduction types

volcanism and aseismic slip in subduction zones the spatial and temporal relationship of volcanism to the occurrence of large earthquakes and convergent plate motion is examined the number of volcanic eruptions per year in a convergent zone is found to be linearly related to the aseismic slip component of plate motion if the aseismic slip rate is low coupling between converging plates is strong then the primary manifestation of tectonic activity is the occurrence of large earthquakes with only infrequent volcanic activity if however the aseismic slip rate is high coupling is weak then there are few large earthquakes and volcanism is the principal manifestation of tectonic activity this model is consistent with the spatial distribution of large earthquakes and active volcanoes in the circum pacific area it is tested by examining the extent of volcanic activity in the rupture zones of the 1952 1973 sequence of earthquakes in the japan kurile islands area the number of volcanic euptions along these zones during the interval between large earthquakes is used to compute the aseismic slip rates for these segments based on the relationship developed in this study the aseismic slip rates so computed agree with those determined from the earthquake history of the area and rates of plate motion themore agreement suggests that in the interval between large earthquakes the aseismic plate motion is manifested in a specific number of volcanic eruptions therefore in areas with adequate historial data it should be possible to use the model developed in this study to monitor volcanic eruptions for long term prediction of large earthquakes less

a short duration of chondrule formation in the solar nebula evidence from 26al in semarkona ferromagnesian chondrules abstract the 26 al 26 mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite semarkona ll3 0 were studied using a secondary ion mass spectrometer their glass or plagioclase portions contain excesses of 26 mg and in two chondrules the 26 mg excesses are well correlated with 27 al 24 mg which demonstrates the in situ decay of 26 al the initial 26 al 27 al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 10 6 and 9 10 6 indicating their short formation duration of less than 1 my if the solar nebula was initially homogeneous in al isotopes the chondrule formation ages are 2 my younger than those of cais our results based on the study of the least metamorphosed uoc are consistent with the previous studies on al rich chondrules that the chondrule formation started at least 2 my after cais alternatively the older records before 2 my were erased by chondrule recycling process it further suggests that the young apparent ages 3 to 5 my after cais for chondrules in type 3 4 uocs are due to the disturbance of the 26 al 26 mg system possibly during parent body metamorphism the result is not consistent with the extended nebular time scale of 5 my and the chondrule formation by planetary processes the ni isotopic analysis of the feo rich olivines in a type ii chondrule in semarkona did not show any detectable excess 60 ni in spite of their high fe ni ratios the upper limit of the initial 60 fe 56 fe ratio of the solar system was estimated to be 3 4 10 7 which is consistent with the previous estimate 0 2 1 9 10 7 from eucrites this result confirms that the 60 ni excess previously observed from cais was not due to the decay of the short lived nuclide 60 fe but a ni isotopic anomaly of nucleosynthetic origin

explosive volcanism and the compositions of cores of differentiated asteroids eleven iron meteorite groups show correlations between ni and siderophile trace elements that are predictable by distribution coefficients between liquid and solid metal in fractionally crystallizing metal magmas these meteorites are interpreted to be fragments of the fractionally crystallized cores of eleven differentiated asteroids many of these groups crystallized from s depleted magmas which we propose resulted from removal of the first partial melt the fe nifes cotectic melt by explosive pyroclastic volcanism of the type envisaged by wilson and keil 8 we show that these dense negatively buoyant melts can be driven to asteroidal surfaces due to the presence of excess pressure in the melt and the presence of buoyant bubbles of gas which decrease the density of the melt we also show that in typical asteroidal materials veins will form which grow into dikes and serve as pathways for migration of melt and gas to asteroidal surfaces since cotectic fe nifes melt consists of about 85 wt fes and 15 wt fe ni removal of small volumes of eutectic melts results in major loss of s but only minor loss of fe ni thus leaving sufficient fe ni to form sizeable asteroidal cores

juan de fuca plate aseismic subduction at 1 8 cm yr volcanic activity in the cascades in historic times suggests that the juan de fuca plate is underthrusting aseismically at about 1 8 cm yr this rate of underthrusting is identical to the rate computed from sediment studies

ionospheric disturbances during low latitude auroral events and their association with magnetospheric processes ionospheric disturbances associated with low latitude auroral events that occurred during geomagnetic storms on october 21 1989 and may 10 1992 are investigated from measurements of the total electron content tec by the u s navy navigation satellite system nnss and ionosonde data from the japanese meridian chain features of the ionospheric disturbances are then associated with the progression of geomagnetic disturbances during the storms after the onset of the main phase of each storm anomalous tec distributions characterized by depressed tec distributions on the high latitude side 35 geographic latitude and an enhanced equatorial ionization anomaly eia on the low latitude side 35 geographic latitude were observed over japan both by the nnss and by the meridian chain of ionosonde stations this enhancement of the eia suggests the penetration of magnetospheric electric fields during the storms corresponding to the anomalous tec decreases in the northern part of japan ionosonde stations in this region observed specific disturbances of ionospheric variation characterized by simultaneous decreases of 0f2 and increases of h f these features of ionospheric variations on the high latitude side can be attributed to the upward escape of ionospheric plasma caused by the heating and evacuation mechanisms which are induced by depletion of the plasmasphere and the resulting access of ring current particles to low latitudes after the appearance of the characteristic ionospheric variations mentioned above low latitude aurorae were observed at the maximum development stage of the dst associated with the recovery of midlatitude geomagnetic horizontal h components prior to the appearance of the low latitude aurorae magnetic disturbances in the auroral region increased with the decrease of midlatitude h components as a natural consequence of magnetic storms however they showed a recovery before the start of the low latitude aurorae while the magnitude of the low latitude magnetic field continued to decrease these observed features of the magnetic disturbances suggest that the position of the auroral oval shifted toward the equator before the appearance of the low latitude aurorae consequently these observations are consistent with the plasmapause and auroral zone moving toward the equator before the onset of low latitude aurorae we conclude that the convection electric field penetrating toward low latitudes causes the enhancement of the eia the shrinkage of the plasmasphere and the penetration of high energy particles toward the low latitude region

nonuniform seismic slip rates along the middle america trench revised estimates of seismic slip rates along the middle america trench are lower on the average than plate convergence rates but match them locally for example oaxaca along the cocos north american plate boundary this can be explained by nonuniformities in slip at points of aseismic ridge or fracture zone subduction for at least 81 yr and possibly several hundred years no major m sub s or 7 5 shallow earthquake is known to have occurred near the orozco fracture zone and tehuantepec ridge areas compared with the average recurrence periods for large earthquakes 33 8 yr since 1898 and 35 24 yr between 1542 and 1979 this suggests that either a large m or 8 4 event may be anticipated at such locations or that these are points of aseismic subduction large coastal terraces and evidence suggesting tectonic uplift are found onshore near the orozco fracture zone the larger discrepancy between plate convergence and seismic slip rates along the cocos carribbean plate boundary is more likely due to decoupling and downbending of the subducted plate we used the limited statistical evidence available to characterize both spatial and temporal deficiencies in recent seismic slip the observations appear consistent with a possible forthcoming episodemore of more intense seismic activity based on a series of comparisons with carefully delineated aftershock zones we conclude that the zones of anomalous seismic activity can be indentified by a systematic automated analysis of the worldwide earthquake catalog m sub b or 4 less

quaternary volcanic front at the junction of the south west japan arc and the ryukyu arc abstract the hohi volcanic zone a 70 by 40 km volcanic graben on kyushu island at the junction of the south west japan arc and the ryukyu arc has been formed since 6 ma to the present quaternary volcanism producing centers along a ne sw trending line the volcanic front started at about 1 5 ma this volcanic front crosses the e w alignment that was built by the pliocene volcanoes of the hohi volcanic zone the quaternary volcanic front is parallel to the depth contours of the present deep seismic plane and is perpendicular to the current direction of subduction of the philippine sea plate the age of formation of the volcanic front coincides with the time when several other tectonic events occurred such as the counter clockwise shift from nnw to wnw in relative convergence direction of the philippine sea plate the volcanic rocks erupted after 1 5 ma are more widely distributed with a larger total volume in the denser seismic zone than in the aseismic zone this suggests that the volcanism on the volcanic front is closely related to seismicity at about 120 km depth the active seismic area may have accelerated to generate a partially molten zone beyond the dragged hydrated peridotite layer above the downgoing slab in comparison to the aseismic area

spatial changes in volcanic and seismic activity prior to great earthquakes spatial relationship between volcanism and seismicity prior to the occurrence of several great interplate earthquakes in the circum pacific area has been examined in order to understand the process of underthrusting in detail the locations of the epicenters of the great earthquakes have been examined in terms of locations of concentrated volcanic and seismic activity in a short time period in order to determine if there is any relationship between these activities and whether such relationships can be used to refine the underthrusting model the study shows that in most cases 1 there is no volcanic activity in the vicinity of the epicenter of the great earthquake and 2 maximum volcanic activity is localized i e volcanoes adjacent to one another exhibit considerable activity in a short time period very little systematic spatial relationship between these three parameters is observed although in most cases there is no volcanic activity at the time of maximum earthquake activity locations of active volcanoes and earthquake activity during the five years prior to the occurrence of the great earthquake do not appear to be a guide to the epicenter of the great earthquake this study therefore suggests that although there is temporal relationship between the occurrence of maximum volcanic and seismic activity and the occurrence of great earthquake there appears to be no systematic spatial relationship between these three parameters

seismicity and aseismic slip along the eltanin fracture zone table al is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 006 2 50 payment must accompany order the seismic history of the eltanin fracture zone on the pacific antarctic ridge for the years 1920 1981 shows that the seismic slip accumulated during this recording interval accounts for only a small fraction less than 10 of the amount of slip predicted by kinematic models of plate motion we propose that the remainder of the plate motion along the transform faults making up the eltanin system occurs as aseismic creep this property singles out the eltanin among transform systems where previous studies have found good agreement between seismic slip and predicted rates of motion the absence of record of any gigantic earthquakes and the geometry of the fault area reduce the likelihood of the eltanin rupturing along its entire length we interpret the eltanin fracture zone in the light of an asperity model involving small well separated asperities its segmented nature and creeplike behavior may be due to the presence of excess magma and possibly local upwelling which could also be involved in the mechanism of generation of the louisville ridge

high time resolution by use of the 26al chronometer in the multistage formation of a cai the allende calcium rich inclusion cai 5241 has been found to contain distinct initial 26 al 27 al in the three consecutive igneous zones that have been identified by extensive petrogenetic studies to have formed in three distinct crystallization events the zones in order of sequence of formation from the petrologic observations are 1 spinel free islands sfi included in 2 a pyroxene spinel rich core src which in turn is included in 3 a melilite mantle mm the initial 26 al 27 al values of these zones are respectively 4 6 5 0 10 5 4 3 10 5 and 3 3 10 5 it is argued that these distinct 26 al 27 al 0 values are not the result of metamorphism but reflect the relative times of formation by crystallization from melts relative to the canonical value of 26 al 27 al 0 5 10 5 we find the following chronology t sfi 0 t src 10 5 yr and t mm 4 10 5 yr the three layer cai is inferred to have sampled a reservoir with an initial uniform 26 al 27 al ratio that decreased in value due to radioactive decay from these observations we conclude that we have resolved time differences of a few hundred thousand years at the very early stages of formation of the solar system the sfi src and mm zones reflect sequential addition of molten cai material which crystallized rapidly without seriously metamorphosing the previously formed material these additions took place over a time of about 400 000 years we believe that these sequential events are not compatible with condensation in a hot region of the solar nebula it is proposed that a scenario involving stages of protoplanetary accretion and of melt generation in protoplanetary sites heated by 26 al might provide a possible source for cais and chondrules

decomposition of multicomponent sea floor data into upgoing and downgoing p and s waves a method for decomposing multicomponent sea floor measurements into upgoing and downgoing p and s waves is presented we assume that a marine survey employing a marine source in the water layer is conducted over a plane layered medium from recordings of the pressure just above the sea floor and the particle velocity vector just below the sea floor decomposition filters can be determined by plane wave analysis the decomposition filter coefficients depend on the p and s wave velocities and the density at the sea bottom we show how to decompose the multicomponent measurements into upgoing and downgoing p and s vertical traction components vertical particle velocity components and horizontal particle velocity components the decomposition filters are applied with good results to synthetic data modeled in a plane layered medium

petrologic and almg isotopic clues to the accretion of two refractory inclusions onto the leoville parent body one was hot the other wasn t abstract two type b refractory inclusions found within several centimeters of each other in the foliated leoville meteorite differ greatly in their styles of deformation textures and mg al isotopic characteristics one is a subspheroidal type b1 with textures similar to those observed previously in many type b1s from other cv3 meteorites and whose deformation fracturing some granulation has been almost entirely brittle in nature its isotopic composition shows 26 mg 24 mg correlated with 27 al 24 mg which although disturbed indicates a lower limit for initial 26 al 27 al of 4 9 10 5 the other inclusion is a highly elongate l w 6 object whose long axis lies in the plane of foliation of the leoville meteorite it has a predominantly metamorphic texture but relict islands of an earlier and more normal type b precursor are preserved in its core the isotopic compositions of the two textural variants in this inclusion are different anorthite in the metamorphic region mostly showing little or no excess 26mg 26 al 27 al 0 6 that would be attributable to in situ decay of 26al whereas anorthite in the relict islands shows well resolved excess 26mg that gives a lower limit for initial 26 al 27 al of 4 6 10 5 the effects of shock metamorphism on the leoville meteorite are clearly seen in both inclusions including brittle deformation and local shock melting but are not responsible for the major textural differences between the two inclusions the most likely origin for the deformed and recrystallized features of the elongate inclusion is that it accreted onto the leoville parent body while still very hot and deformed into its present shape at the instant it impacted if the resetting of the al mg system occurred during this event it must have taken place at least 3 3 million years after formation of the precursor inclusion the possibility that other deformed leoville components accreted while hot should not be discounted

reply to h acharya s comments on seismic potential associated with subduction in the northwestern united states subduction in the northwestern united states presents us with a dilemma although there is good evidence of 3 to 4 cm yr of convergence between the juan de fuca and north american plates the occurrence of either historic or instrumentally recorded shallow thrust earthquakes is remarkably low why aren t there more earthquakes aseismic slip along the entire plate boundary provides a convenient explanation for this dilemma aseismic slip appears to be the predominant mode for plate interaction for many subduction zones however the juan de fuca subduction zone is clearly different from the most common class of aseismic subduction zone that is characterized by the subduction of very old oceanic lithosphere heaton and kanamori 1984 we noted that in general the subduction of young lithosphere is characterized by strong interplate seismic coupling the juan de fuca subduction zone can be considered as an end member in that it involves some of the youngest subducted lithosphere observed anywhere this in itself suggests that the juan de fuca subduction zone belongs in a class perhaps aseismic by itself however there are several other localities where comparably young crust appears to be subducting these are southern chile between 42 and 45 south latitude colombia near 2o north latitude and the rivera plate off western mexico all of these regions are seismically active and in the case of colombia and southern chile have involved earthquakes with energy magnitudes of 8 8 1906 and 9 5 southern half of the 1960 rupture zone respectively furthermore these regions of colombia and southern chile do not have bathymetric trenches and there is no significant seismic activity deeper than 100 km observed on their benioff wadati zones there is also evidence that these regions have experienced significant periods of seismic quiescence the noaa catalog shows a remarkable absence of shallow activity between 41o and 45 south latitude along the chile trench for at least 30 yr prior to the 1960 m w 9 5 chilean earthquake unfortunately this catalog is not sufficiently complete to allow a comparison of seismicity at small magnitude earthquakes however at a magnitude cutoff of 6 the rate of seismicity in the 50 yr preceding the 1960 earthquakes in the region between 41o and 45o south latitude seems comparable to that reported for the juan de fuca subduction zone s 150 yr history the juan de fuca convergence rate is about one third that of southern chile

vector acoustic reverse time migration of volve ocean bottom cable data set without up down decomposed wavefields abstractmethods for wavefield injection are commonly used to extrapolate seismic data in reverse time migration rtm injecting a single component of the acoustic field for example pressure leads to ambiguity in the direction of propagation each recorded wavefront is propagated both upward and downward and spurious or ghost reflectors are created alongside real reflectors in the subsurface image thus wavefield separation based on the combination of pressure and particle velocity data is generally performed prior to imaging to extract only the upgoing field from multicomponent seabed or towed marine seismic recordings by instead combining vector acoustic va data with monopole and dipole type propagators in the extrapolation of shot or receiver gathers we show that wavefield separation or deghosting can instead be performed on the fly at limited additional cost this strategy was successfully applied to a line of a north sea ocean bottom cable data set acquired over the volve field we t

comparison of seismicity parameters in different subduction zones and its implications for the cascadia subduction zone the cascadia subduction zone is a young subduction zone with associated volcanic activity but with no history of large thrust earthquakes associated with it as is characteristic of other subduction zones this has led to questions about the potential for great earthquakes associated with this subduction the seismic potential associated with this subduction has been investigated in this study by examining and comparing the number of shallow earthquakes with magnitude 6 which have occurred in different subduction zones since 1910 this examination shows that all subduction zones exhibit a higher level of activity than cascadia furthermore two of these zones with similar rates of plate motion and age at subduction the rivera plate and southwest japan have not only experienced great earthquakes but show a higher rate of activity than the cascadia zone the activity rate in subduction zones with predominantly aseismic slip is also higher than the activity rate in the cascadia zone earthquake activity in the rupture zones of great earthquakes both before and after the occurrence of great earthquakes is higher than the activity rate in the cascadia zone these results therefore suggest that the low rate of activity in the cascadia zone is unusual and supports the idea that the subduction here is different and that perhaps the potential for great earthquakes cannot be extrapolated simply on the basis of certain similarities between the cascadia zone and other subduction zones with a history of great earthquakes the low rate of earthquake activity in the cascadia zone can however be explained on the basis of the combined effect of low rate of plate motion significant aseismic subduction and young age at subduction the activity rate in the cascadia zone is consistent with the activity rates in other subduction zones if it is assumed that much of the plate motion is accommodated aseismically the seismic slip rate associated with this subduction zone is a small fraction of the rate of plate motion and therefore even if the potential for great earthquakes may exist the return period for great earthquakes will be much longer 2 or 3 times than in other zones with similar characteristics such as the rivera plate or southwest japan

an introduction to the sangihe arc volcanism accompanying arc arc collision in the molucca sea indonesia in the molucca sea region of northeastern indonesia the sangihe and halmahera arcs are presently in the process of colliding the earth s only example of a collision between facing volcanic arcs the collision event is more advanced in the northern molucca sea where back arc thrusting occurs along the cotobato and philippine trenches and volcanic centers are inactive and dissected this paper describes the sangihe arc the western arc of the collision the sangihe are is approximately 500 km in length extending from the ne tip of sulawesi to mindanao philippines the arc comprises over 25 quaternary volcanic centers the eight presently active volcanoes being confined to the southern half of the arc the active volcanic front lies 100 110 km above the top of the west dipping benioff zone and volcanoes extend up to 70 km behind the front thereby defining a volcanic arc overlying earthquakes 100 to 180 km deep rock types range from basalt to rhyolite andesites are dominant pyroxene andesites are common on sulawesi and at or near the volcanic front from ruang to buhias pahepa and buang to mamanuk hornblende andesites are common at the volcanic front between mahengetang and sangihe island and in centers located well behind the volcanic front tholeiitic th suites defined on the basis of relative degrees of iron enrichment are confined to the southern volcanic front lack hornblende are low k k55 1 05 have low rb zr ratios 0 10 0 17 low ba y ratios 3 4 4 9 low rb cs ratios 8 14 and have the highest 87sr 86sr ratios 0 7039 0 7042 calcalkaline ca suites occur throughout the arc are low to high k k55 0 6 2 0 have variable rb zr ratios 0 15 0 65 and ba y ratios 4 2 12 4 high rb cs ratios 14 82 and low 87sr 86sr ratios 0 7035 0 7040 zr and y distribution coefficients are lower in th suites dzr 0 0 dy 0 2 0 5 than in ca suites dzr 0 0 0 8 dy 0 3 1 2 ti initially increases with increasing sio2 in some th suites while decreasing with sio2 in all ca suites with the exception of bukide ti zr 138 ti zr ratios of basalts exceed 100 only in th suites 76 130 lil element contents and lil hfs element ratios increase 2 to 3 times across the are away from the volcanic front and 2 to 3 times from south to north along the arc 87sr 86sr ratios decrease away from the volcanic front the south to north increase in lil element contents and lil hfs element ratios without corresponding changes in sr isotopic compositions are interpreted as resulting from decreasing degrees of partial melting northward this reflects the gradual cessation of volcanism in the north due to collision

a petrographic chemical and isotopic study of calcium aluminum rich inclusions and aluminum rich chondrules from the axtell cv3 chondrite petrographic compositional and isotopic characteristics were studied for three calcium aluminum rich inclusions cais and four plagioclase bearing chondrules three of them al rich from the axtell cv3 chondrite all seven objects have analogues in allende cv3 and other primitive chondrites yet axtell like most other chondrites contains a distinctive suite of cais and chondrules in common with allende cais cais in axtell exhibit initial 26 al 27 al ratios 26 al 27 al 0 ranging from 5 10 5 to 2 ma after the first cais as in other cv3 chondrites some objects in axtell show evidence of isotopic disturbance axtell has experienced only mild thermal metamorphism 600 c probably not enough to disturb the al mg systematics its cais and chondrules have suffered extensive metasomatism probably prior to final accretion these data indicate that cais and chondrules in axtell and other meteorites had an extended history of several million years before their incorporation into the axtell parent body these long time periods appear to require a mechanism in the early solar system to prevent cais and chondrules from falling into the sun via gas drag for several million years before final accretion we also examined the compositional relationships among the four plagioclase bearing chondrules two with large anorthite laths and two barred olivine chondrules and between the chondrules and cais three processes were examined 1 igneous differentiation 2 assimilation of a cai by average nebular material and 3 evaporation of volatile elements from average nebular material we find no evidence that igneous differentiation played a role in producing the chondrule compositions although the barred olivine compositions can be related by addition or subtraction of olivine methods 2 and 3 could have produced the composition of one chondrule axch 1471 but neither process explains the other compositions our study indicates that plagioclase bearing objects originated through a variety of processes

volcanic activity and large earthquakes abstract several investigators have noted an increase in volcanic activity prior to several large interplate earthquakes in the circum pacific area from these data i have examined the temporal relationship between the beginning of this increased volcanic activity and the occurrence of large earthquakes i find that the time year of the beginning of enhanced volcanic activity depends on the magnitude of the subsequent large earthquake for earthquakes with magnitude 7 5 the volcanoes which exhibit this enhanced activity are situated on the landward side of the rupture zone of the large earthquake and this study therefore suggests a casual physical relationship between the two phenomena i also find that this increase in volcanic activity occurs a few years before an increase in the occurrence of smaller earthquakes noted by several invesigators in the rupture zones of large earthquakes

equatorial density depletions observed at 840 km during the great magnetic storm of march 1989 early on march 14 1989 a thermal plasma probe on the defense meteorological satellite program dmsp f9 spacecraft detected extensive and dramatic decreases in the ion density at 840 km near 2130 lt during two consecutive transequatorial passes over south america the order of magnitude decreases in the ion density extended more than 4000 km along the satellite track the depletions were accompanied by upward and westward plasma drifts both in excess of 100 m s their onsets and terminations were marked by extremely sharp density gradients dmsp f9 observed no similar depletions over the atlantic during preceding orbits a partial depletion was detected over the eastern pacific during the following orbit the dmsp f9 ground track passed slightly west of a brazilian total electron content tec station and two brazilian ionosondes during the first depletion encounter the tec fell far below normal during the night of march 13 14 the ionosonde measurements indicate that in the hour after sunset before dmsp passed through the depletions the f2 layer rose rapidly and disappeared but at the time of the first depletion encounter hmf2 was decreasing over one of the stations the dmsp f8 satellite which orbits in the dawn dusk meridian made related measurements on march 13 and 14 crossing the equator at dust on march 13 at the same longitude where dmsp f9 encountered the first depletion dmsp f8 detected upward and westward drifts but it measured extremely large rather then depleted ion densities during two dawn passes over the eastern pacific on march 14 dmsp f8 observed depletions somewhat similar to those detected by dmsp f9 large westward drifts accompanied the depletions detected by dmsp f8 it is quite probable that the morningside depletions detected on march 14 are remnants of those detected earlier by dmsp f9 in the evening sector we develop a phenomenological model reconciling dmsp f8 f9 and ground based measurements our calculations show that rapid upward drifts sustained for several hours can produce depletions in the equatorial ion density with sharp gradients at their high latitude boundaries consistent with the data we discuss possible contributing mechanisms for generating these upward drifts these include direct penetration of the magnetospheric electric field to low latitudes the electric fields generated by the disturbance dynamo and the effects of conductivity gradients near the dusk terminator and the south atlantic anomaly

using even terms of the scattering series for deghosting and multiple attenuation of ocean bottom cable data inverse scattering multiple attenuation isma is a method of removing free surface multiple energy while preserving primary energy the other key feature of isma is that no knowledge of the subsurface is required in its application i have adapted this method to multicomponent ocean bottom cable data i e to arrays of sea floor geophones and hydrophones by selecting a subseries made of even terms of the current scattering series used in the free surface multiple attenuation of conventional marine surface seismic data streamer data this subseries approach allows me to remove receiver ghosts receiver side reverberations and free surface multiples source side reverberations in multicomponent obc data i have processed each component separately as for the streamer case my obc version of isma preserves primary energy and does not require any knowledge of the subsurface moreover the preprocessing steps of muting for the direct wave and interpolating for missing near offsets are no longer needed

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

anatomy of an allende coarse grained inclusion inclusion 5241 is a petrographically complex coarse grained cai from the allende meteorite containing a core with two petrographically and chemically distinct units the core is surrounded by a melilite mantle similar to that described for allende type b1 inclusions which consists of large millimetersized crystals the two different units of the core are 1 spinel free islands consisting of melilite fassaite and minor anorthite 2 spinel rich areas with major fassaite and reversely zoned melilite in this unit spinel occurs in fassaite melilite and in densely packed belts surrounding the spinel free islands mineral chemistries of melilite and fassaite in the spinel free islands are different from those in the spinel rich areas melilite in the spinel free islands is unzoned and displays a very narrow chemical variation 10 mol within individual crystals ranging between ak 52 and ak 68 in all islands they also contain considerable amounts of na melilite molecule as much as 2 7 mol in contrast melilite in the spinel rich areas is reversely zoned and barren of na melilite the composition varies between ak 43 and ak 62 with the lowest ak content within a zone decorated with fassaite and spinel inclusions zoning in individual melilite crystals in the mantle is not continuous as in allende type b1 inclusions but is rhythmic outwards from ak 30 to ak 35 near the core mantle boundary followed by a continuous decrease with the lowest value near the center of the melilite crystals ak 25 the ak content rises again and reaches a value of ak 28 at the rim of the inclusion fassaite in the spinel free islands is typically concentrically zoned with ti al rich cores and ti al poor rims the concentration of ti 3 in some fassaite crystals in the islands appears to be lower than in the spinelrich areas zoning of fassaite in the spinel rich areas is nonsystematic and the tio 2 variations are considerable 2 12wt bulk compositions of the spinel free islands plot inside the type b cai field but considerably below the fo an ge plane because they are spinel deficient in contrast compositions of the spinel rich areas plot outside the type b cai field close to pyroxene saturated compositions stolper 1982 textural relations and mineral chemistries in the spinel free islands spinel rich areas and melilite mantle cannot be explained by gas solid condensation simple stage crystallization from a refractory silicate liquid or evaporative loss by distillation but suggest a series of complex events including the capture of solid spinel free bodies by a refractory liquid the melilite mantle may have formed by crystallization from a liquid which enveloped the complex core

recent research on the physical aspects of earthquakes abstract recent developments in the field of physical aspects of earthquakes which encompasses in present day terminology the analysis of focal dynamics and of catastrophic effects of earthquakes are reviewed in particular individual sections of this review deal with the earthquake source effects of earthquakes on the ground the geographic and temporal distribution of earthquakes the characterization of seismic risk earthquake prediction and with the artificial release of earthquakes in this instance the review supplements earlier information by the author 1975 by new data published mainly between 1975 and 1984

frequency domain direct waveform inversion based on perturbation theory s u m m a r y adirect waveform inversion based on perturbation theory is proposed to delineate a subsurface velocity structure from seismic data this technique can directly compute the difference between the actual subsurface velocity and an initial guess of the velocity while full waveform inversion updates the velocity model in the directions of reducing the data residual unlike full waveform inversion using the steepest descent method the direct waveform inversion does not require a proper step length to iteratively update the velocity model we present an algorithm for thewaveform inversionmethod in the frequency domain and numerical examples demonstrating how the inversion method can reconstruct subsurface velocity structures using surface seismic data the time domain seismograms synthesized in the inversion procedure match the corresponding shot gather seismograms of field data