magnitude scale and quantification of earthquakes despite various shortcomings the earthquake magnitude scale is one of the most fundamental earthquake source parameters to be used for catalogs although use of a uniform scale is desirable it is not always possible because of changes in instrumentation the data reduction method and the magnitude formula the station distribution etc as a result various magnitude scales have been developed and are currently in use recent developments in seismometry and earthquake source theories provide more quantitative source parameters than the magnitude in order to maintain continuity and uniformity of the data it is important to relate these magnitude scales and the new parameters in view of this importance relations between different magnitude scales are examined with an emphasis on the difference in the period of the waves used for the magnitude determination use of several magnitude scales determined at different periods provides a convenient method for characterizing earthquakes the moment magnitude can be used to quantify both shallow and deep earthquakes on the basis of wave energy radiated and provides a uniform scheme

moment tensors and source processes of three deep sea of okhotsk earthquakes abstract three large deep earthquakes beneath the sea of okhotsk are studied using the relative amplitudes of the direct p phase and the surface reflections pp and sp observed on teleseismic long period wwssn seismograms use of this comprehensive dataset provides a redundancy of information which allows possible deviation of the source mechanism from the double couple model to be studied over a large part of the focal sphere in a region where previous workers have postulated complex source mechanisms including implosive precursors and multiple events focal mechanism solutions derived from the relative amplitudes are used to create synthetic seismograms which further assist the modelling of the source processes of these earthquakes one of the three earthquakes occurred close to a sequence of complex earthquakes in 1970 1971 and comprises a number of discrete but related subevents the two main subevents with similar source mechanism orientations and the latter subevent probably triggered by the first the other two earthquakes which occurred further southwest are similar to each other and their seismograms display features characteristic of the finite size and rupture propagation of the source which enable the source size geometry and rupture velocity to be estimated the source moment tensors of all three earthquakes show no significant deviation from a simple double couple and show no evidence of a volumetric component

intermediate depth seismicity in the western mediterranean unrelated to subduction of oceanic lithosphere there are three zones in the gibraltar area where intermediate depth earthquakes occur down to 150 km two of these the gulf of cadiz and the western alboran sea are oceanic areas which could be associated with the subduction of oceanic lithosphere our present measurements of arrival times of p and s waves show however that the third zone beneath the high atlas is a typical intracontinental chain where it is not likely that oceanic lithosphere has been subducted during the past 20 million years

geometry of the subducted plate near kodiak island and lower cook inlet alaska determined from relocated earthquake hypocenters abstract using observations from 28 seismograph stations in southern alaska we have used the jhd method to relocate 341 well recorded events with focal depths beneath about 40 km which occurred between 56 4 n and 60 5 n and between 151 6 w and 156 5 w these hypocenters show that the wadati benioff zone has a dip of about 45 however its strike changes by about 15 beneath lower cook inlet at about 59 n near where the volcanic line also bends northward near this bend there is also a distinctly higher level of seismic activity than in the area to the southwest of 59 n a few events are situated about 20 km beneath the main wadati benioff zone in cook inlet and a focal mechanism we have determined for one of these differs from other mechanisms for events in this area focal mechanisms available from previous work show that at intermediate focal depths the area between 59 n and 60 5 n is controlled by horizontal north south compression and as such differs from the region to the southwest relying primarily on the observed change in strike and apparent thickness of the wadati benioff zone we interpret these data to suggest that the subducted lithophere separates into two distinct segments as it subducts beneath cook inlet because the direction of subduction is not perpendicular to the volcanic arc the events situated beneath the wadati benioff zone in cook inlet may be part of the southwestern plate segment although the geometry and available focal mechanisms for events beneath cook inlet are similar to what would be expected in a double wadati benioff zone we suggest instead that these observations result because we are at the boundary between two adjacent segments of the subducting plate

systematic analysis of deep seismicity 200 centroid moment tensor solutions for earthquakes between 1977 and 1980 summary a systematic study of deep seismicity is performed using waveform data from the gdsn between 1977 and 1980 and the centroid moment tensor method of dziewonski et al two hundred new cmt solutions are found for events deeper than 100km their moments obtained using the same algorithm range between 7 x and 1 4 x lo2 dyne cm a factor of 20 000 a statistical analysis of the cmt parameters provides important conclusions among these the shift in origin time for deep events is found to be almost independent of the seismic moment providing an indication of the complexity of deep earthquakes the deviation from the double couple model appears to be correlated to the state of stress of the subducting slabs mechanisms showing down dip compression are generally dominated by the eigenvalue corresponding to the compression axis and down dip tension correlates with the predominance of the tensional eigenvalue comparison with moment tensors obtained by other authors using different waveform data shows good agreement between different methods comparison with published focal plane mechanisms confirms the precision and reliability of the cmt method examples of cmt solutions used to identify patterns of seismicity associated with deep fault planes are shown

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

observations of body wave multipathing from broadband seismograms evidence for lower mantle slab penetration beneath the sea of okhotsk we present observations of anomalously broad and complex sh waveforms from two deep focus earthquakes in the southern kurils to digital world wide standard seismograph network and regional seismic test network stations in north america by making use of broadband three component seismograms and relying on a variety of phases one can limit the class of possible explanations for these phases in particular it is shown that the anomalous arrivals are not due to source complexity near receiver structure a triplication from a lower mantle velocity gradient a low q zone in the mid lower mantle or shear wave splitting we show that these arrivals correspond to relatively fast paths and have enhanced amplitudes indicating that they are most probably caused by multipathing due to interaction with a high velocity lower mantle feature below the source region we examine the hypothesis that this feature may be a lower mantle extension of the kuril slab ray tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the north american stations used in this study for a realistic slab model observations of reversed polarity arrivals are consistent with such an explanation where it appears that two rays comprising the arrival emerge from different points on the focal sphere one test of this explanation is that slab induced multipathing should not be seen along back of slab oceanic azimuths examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the sh waveforms identification of slab induced multipathing associated with deep focus events has two important consequences first it appears to require slab penetration beyond the 650 km discontinuity at least 300 400 km into the lower mantle under the sea of okhotsk second these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities low q zones near receiver structure or source complexity

kiyoo wadati and early research on deep focus earthquakes introduction to special section on deep and intermediate focus earthquakes this special section of the journal of geophysical research is dedicated to kiyoo wadati this paper evaluates his contribution to our knowledge of geophysics and especially deep earthquake phenomena in at least three areas wadati wrote several papers before 1936 which profoundly influenced modern geophysics first his work provided the first convincing evidence that deep earthquakes existed second he was a leader in the construction of travel time tables and the determination of mantle velocity structure finally he published the first accurate description of the inclined planar zone of deep earthquakes which extends from trenches beneath volcanic island arcs wadati s work strongly influenced research of many other scientists both before and after world war ii particularly jeffreys gutenberg and benioff several of the questions raised by the research of wadati and others before 1940 are still unanswered today

the depths of the deepest deep earthquakes the maximum depth of seismic activity is a fundamental observation providing a constraint on models of mantle dynamics although most recent investigations of mantle seismicity and dynamics state that seismicity extends to about 700 km focal depths of 720 km have been reported for large earthquakes as well as depths greater than 800 km for small events we have examined focal depths of events in several catalogs and conclude that 1 the deepest reliable focal depths are at about 670 680 km and events with focal depths beneath 600 km occur in several widely separated geographic regions 2 observations supporting focal depths exceeding 680 km are usually few poor or inconsistent 3 the reduction of seismic activity beneath about 650 km is quite abrupt however we cannot determine whether activity stops completely at 670 680 km or the maximum size of events decreases gradually beneath 650 km with magnitudes of 6 5 7 0 occasionally occurring at 650 km magnitudes of 5 5 6 0 at 680 km 4 0 4 5 at 695 km etc several models can explain the absence of seismic activity in the lower mantle two models are consistent with the abrupt termination of activity at about 650 680 km in widely separated areas a barrier which resists penetration by the subducting lithosphere or a phase transition which modifies the properties of the subducted material as it goes into the lower mantle

remote triggering of deep earthquakes in the 2002 tonga sequences it is well established that an earthquake in the earth s crust can trigger subsequent earthquakes but such triggering has not been documented for deeper earthquakes models for shallow fault interactions suggest that static permanent stress changes can trigger nearby earthquakes within a few fault lengths from the causative earthquake1 2 3 whereas dynamic transient stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances4 5 6 7 8 here we present a detailed analysis of the 19 august 2002 tonga deep earthquake sequences and show evidence for both static and dynamic triggering seven minutes after a magnitude 7 6 earthquake occurred at a depth of 598 km a magnitude 7 7 earthquake 664 km depth occurred 300 km away in a previously aseismic region we found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock but the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible thus suggesting dynamic triggering the origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone we propose that these events are triggered by transient effects in regions near criticality but where earthquakes have difficulty nucleating without external influences

the 1996 june 17 flores sea and 1994 march 9 fiji tonga earthquakes source processes and deep earthquake mechanisms summary source parameters of the 1996 flores sea and 1994 fiji tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broad band seismograph stations both events consisted of several subevents models to approximate the spatial and temporal extent of the source process include point sources propagating point sources and a combination of these for the flores sea event rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of p wave pulses in agreement with the findings reported by other investigators our preferred model suggests bilateral rupture propagation it consists of four point sources that have variable double couple radiation patterns and source time histories and explains well the large compensated linear vector dipole clvd component inferred from the harvard centroid moment tensor cmt solution the main moment release in the fiji tonga event lasted only about 15 s our best model consists of two point sources with a total moment release of 2 8 1020 n m rupture propagated subhorizontally from the nucleation point to the north the termination of rupture was located about 40 km to the north of rupture initiation the inferred velocity of moment release in the flores sea and fiji tonga events was 3 5 km s 1 a value which is higher than that inferred for the great bolivian earthquake of june 1994 other derived source parameters static stress drop radiated seismic energy and maximum seismic efficiency are also significantly different from those inferred for the 1994 bolivian event suggesting that deep earthquake processes do not follow an easily detectable common mechanism

a method of studying variations in the clustering property of earthquakes application to the analysis of global seismicity we propose a formal algorithm for identification of aftershocks following a main event our approach is based on the analysis of the local statistics the number of events following the initial earthquake displayed as a function of distance and time by comparing this distribution with the random catalog we establish for several magnitude ranges spatiotemporal thresholds for detection of aftershocks high confidence thresholds correspond to the limit at which the number of events derived from the actual catalog corrected for the background seismicity exceeds by a factor of 10 the expected level of background seismicity marginal confidence thresholds correspond to the spatiotemporal boundary along which these two values are approximately equal after several iterations the final thresholds are derived the swarming parameter is defined as the number of aftershocks with a cut off lower magnitude range following an initial earthquake of certain magnitude if a correction for the recurrence law is applied we find that the swarming parameter is magnitude independent this allows us to apply this technique to the analysis of global seismicity using a catalog derived from the isc bulletin for the years from 1964 to 1975 we determine for shallow earthquakes the average values of the swarming parameter in 9 9 cells a global map produced in this manner shows a very wide range of variations in the swarming property undoubtedly reflecting variations in the local geological conditions triple junctions are among the areas with the highest swarming values at the present time there is an insufficient amount of data to study regional variations in the swarming property of deep earthquakes considering the clustering as a function of depth alone we find detectable swarming for two depth ranges from 100 to 250 km and from 450 to 650 km not a single aftershock has been detected among nearly 200 earthquakes withmb 5 in a depth range from 250 to 450 km the average swarming parameter for earthquakes deeper than 100 km is 10 times smaller than that for the shallow earthquakes

aftershocks and temporal clustering of deep earthquakes this paper evaluates how the focal depth the geographic location and magnitude affect the incidence of aftershocks and the occurrence of temporal clustering of nearby events for selected deep earthquakes h 70 km in the catalog of the international seismological centre for comparison we also evaluate shallow h 70 km and deep earthquakes for which locations and moment tensors are reported by the harvard group the primary conclusions are as follows 1 the incidence of aftershocks depends strongly on focal depth with aftershocks being less common for events between 100 and 450 km depth than for shallower or deeper events events with focal depth shallower than 40 km have a higher incidence of aftershocks and more pronounced temporal clustering than do deeper events 2 for the data available at present the incidence of aftershocks and amount of temporal clustering for deep events are independent of geographic region except for dependence caused by regional differences in the focal depths of events 3 deep earthquakes with large magnitudes are more likely to have aftershocks than are smaller magnitude events in accordance with models where the number of aftershocks is a poisson random variable that is a function of magnitude however several large events are anomalous in that they have many more aftershocks or many fewer aftershocks in the catalog than predicted by the model

statistical methods for comparing directions to the orientations of focal mechanisms and wadati benioff zones we propose simple graphical methods for evaluating whether a set of direction vectors are clustered with respect to particular directions axes or planes as examples of these techniques we present data concerning the relationship between 1 aftershock initial event geometry and nodal planes of focal mechanisms 2 aftershock initial event geometry and the plane of the wadati benioff zone and 3 b axes of focal mechanisms and the plane of the wadati benioff zone

slab pull and the seismotectonics of subducting lithosphere this synthesis links many seismic and tectonic processes at subduction zones including great subduction earthquakes to the sinking of subducted plate earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity providing details on how subduction occurs compression is localized across a locked interface thrust zone because both the ridge push and the slab pull forces are resisted there the slab pull force increases with increasing plate age thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate the 1960 chile earthquake mw 9 5 the largest earthquake to occur in historic times began its rupture at an interface bounding oceanic plate 30 m y old however this rupture initiation was associated with the locally oldest subducting lithosphere weakest coupling the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere terminating near the subducting chile rise prior to a great subduction earthquake the sinking subducted slab will cause increased tension at depths of 50 200 km with greatest tension near the shallow zone resisting plate subduction plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260 350 km provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle this explains k mogi s observations of m 7 thrust earthquakes at depths of 260 350 km immediately downdip and within 3 years prior to five great shallow earthquakes of northern japan the slab pull model explains the lower layer of double seismic zones as due to tension from the deeper sinking plate and the upper layer as due to localized in plate compression as plate motion is resisted by the bounding mantle just downdip of the interface thrust zone there occurs an aseismic 20 50 dip increase of subducted plate this slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone

seismic slip and down dip strain rates in wadati benioff zones the rate of accumulation of seismic moment in wadati benioff zones is used to estimate strain rates in subducting slabs that are sinking through the asthenosphere between depths of 75 and 175 kilometers a typical down dip strain rate is about 10 15 per second which implies that slabs in this depth range typically accumulate strain of order 10 1 this result is in accord with geometrical arguments that subducted slabs must experience large membrane strains to deform to their observed shapes mantle seismicity repeated catastrophic shear failure is apparently a primary mechanism by which large membrane strains accumulate in the cold cores of subducting slabs slabs are penetratively deformed and they have low flexural rigidity compared to oceanic plates at the earth9s surface

ruptures of deep focus earthquakes in the north western pacific and their implications on seismogenesis summary the high resolution of broad band seismic data is highly effective in studying ruptures associated with deep focus earthquakes in this study we report results from an analysis of p and sh waveforms from eight of the largest deep focus earthquakes that occurred between 1987 and 1992 along the north western pacific in this region broad band data recorded at close in distances directly above the source zone offer an unusual source receiver geometry that complements observations at teleseismic distances among the eight earthquakes studied six showed clear indications of directivity the best constraint on the extent of rupture came from the steeply dipping izu bonin wadati benioff zone where subhorizontal ruptures typically originate in the interior of subducted slab and propagate toward the top at high angles in particular rupture of the 1988 september 7 event consists of two en echelon line sources at a depth of approximately 480 km the overall rupture length of this earthquake constrains the minimum thickness of the seismogenic zone to be 27 12 km at this depth our estimated thickness of the seismogenic zone seems remarkably close to the maximum thickness 35 km predicted by the metastable olivine wedge model of seismogenesis this model interprets deep focus earthquakes as a result of transformational faulting within a region bounded approximately by the 700 c isotherm the extent of rupture during the largest events in this region seems to be only slightly larger than the 20 25 km spacing between the two layers of a double seismic zone found at depths between 300 and 400 km in the same general area beneath the island of sakhalin a rupture speed greater than that of the shear wave is not required for the large deep focus earthquake 615 km of 1990 may 12 almost all complexity in observed waveforms for this event can be explained by a northward propagating subhorizontal rupture composed of only two subevents

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

dynamic stress drops of moderate earthquakes of the eastern aleutians and their relation to a great earthquake short period wwssn records are used to calculate dynamic stress drops of moderate earthquakes m sub b 5 5 to 6 1 in the eastern aleutians alaska for short period pulses where scattering effects often distort the pulse shape estimates of dynamic stress drop using the formula of boatwright 1980 have fewer sources of uncertainty than does the formula for static stress drop given by brune 1970 1971 earthquakes of several tectonic settings within the subduction zone were studied shallow 5 to 8 km normal faulting events near the trench earthquakes in the main thrust zone 30 to 50 km depth and intermediate depth 100 to 200 km earthquakes in the benioff zone dynamic stress drops of the intermediate depth earthquakes ranged from 20 to 70 bars while the stress drop of earthquakes in the main thrust zone varied over a considerably larger range 10 to 300 bars the two normal events had stress drops around 100 bars evidence from the great 1964 alaska earthquake and its aftershocks also suggests an inhomogeneous stress field in the main thrust zone which may produce large amounts of high frequency radiation when ruptured by a large earthquake 27 references 16 figures 4 tables

tidal triggering of intermediate and deep focus earthquakes tidal stress changes within the earth due to the gravitational attraction of the sun and moon can exceed 20 mbar h whereas tectonic stress rates are of the order of 0 2 mbar h although the absolute magnitude of tidal stresses is less than 1 of the typical stress drops observed in earthquakes a correlation between the tidal stress history on a fault plane and the time of failure should exist if 1 tidal and tectonic stresses are combined in a simple manner 2 the tectonic stress buildup over time is smooth and 3 failure occurs immediately upon attainment of some critical stress to search for possible tidal triggering of intermediate and deep focus earthquakes 7359 events were selected from a global catalog and subdivided into 30 regions each event was assigned a tidal phase relative to both the semidiurnal and biweekly tidal stress curves at the time of occurrence all tidal phase distributions were checked for a preferred phase orientation using the rayleigh test and for nonrandomness using the chi square test results of this analysis for the semidiurnal earth tide show that three regions have a preferred phase direction and four regions display nonrandomness significant at the 90 confidence level which would be expected for this number of random data sets the biweekly tide when normalized for the unequal distribution of biweekly phase over time after correction for clustering of events in some of the data sets shows only three non random data sets at the 95 level as defined by the chi square test thus intermediate and deep focus earthquakes in most subduction zones occur independently of both the semidiurnal and biweekly tidal stress cycles and we may conclude that failure below 70 km does not occur at the instant a stress curve the simple sum of tectonic and tidal stresses reaches the yield strength of subcrustal rocks this implies that either tectonic stress rates prior to failure are not smooth and are unaffected by predictable sources of stress or that failure does not necessarily occur when a certain critical stress is reached these may in turn be due to some time delay characteristic of deep failure to the stress history of the material to the diffusion of fluids or to the mode of failure itself at present a unique and unexplained phenomenon at such high temperatures and pressures

spherical shell tectonics buckling of subducting lithosphere abstract the lithospheric plate is a spherical shell rather than a plane plate a spherical shell must buckle when it is bent inward we examined the possibility of lithospheric buckling upon subduction the lithosphere with its subducted portion is simulated by a hemispherical elastic shell bent inward at its circumferential edge by a uniform radial load the buoyancy force acting on the lithosphere seaward of the trench is simulated by clamping the shell along a parallel the deformable portion between the loaded and clamped edges corresponds to the subducting slab of lithosphere buckling analyses were made using the techniques of a linear stability analysis and a finite element method the wavelength of buckling depends on the thickness of the shell and the length of its deformable portion a longer wavelength is associated with a thicker shell and a longer deformable portion by scaling the shell thickness and the length of the deformable portion to the elastic thickness of lithosphere and the length of subducting slab respectively the wavelength of buckling can be compared favourably to the length of one unit of arcuate trench in a chain like continuation of island arcs the load to be applied for initiation of buckling is called the critical load a lower critical load is associated with a thinner shell with a longer deformable portion the excess weight of subducting slab provides a load large enough to initiate lithospheric buckling at a relatively early stage of subduction radial displacement of the circumferential edge at the critical state is an order of magnitude smaller than the depth of the leading edge of the wadati benioff zone indicating that most of the present subducting lithospheres are under a postbuckling state undulation of the shell in the postbuckling state is not purely sinusoidal but a successive continuation of arcs with cusps in between invoking the continuation of arcuate deep sea trenches with cusps at their junctions the cuspate feature of island arc chains is thus a natural consequence of lithospheric buckling and does not require any such irregularities as seamounts colliding with a trench collision of seamounts however can aid buckling greatly if they are aligned along a trench at an interval not very different from the inherent buckling wavelength

a hybrid method for calculating the radiated wave energy of deep earthquakes abstract the energy radiated as waves is a parameter vital to the understanding of earthquake rupture physics calculations of this energy however are difficult and wrought with uncertainty we developed a new hybrid method for the calculation of radiated wave energy and other key seismic parameters the method calculates these parameters in an internally consistent and objective manner allowing for robust comparison between parameters and across earthquakes in addition the method does not require deconvolution or filtering of data thus results contain all recorded high frequencies we use the method to calculate source time functions seismic moment duration and radiated wave energy for 19 recent deep depth 292 km earthquakes ranging in size from m w 6 5 to 8 2 we compute effective seismic efficiency for the three largest events the principle result is that the effective seismic efficiencies are less than 0 3 if earthquake rupture adhered to the orowan model then the average frictional stress during rupture is equal to the final shear stress on the fault plane and the seismic efficiency is 1 but since the measured seismic efficiency is less than 1 the rupture process of these large deep earthquakes partitions more energy into friction and the average frictional stress during rupture is higher than the final fault plane stress

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

deep earthquake rupture histories determined by global stacking of broadband p waveforms 1 we conduct a systematic investigation of time histories of moment release in deep earthquakes whose physical mechanism remains unknown we have constructed source time functions stfs of 111 large deep earthquakes depth 100 km and mw 6 4 by stacking p waveforms from broadband seismograms recorded by the global seismographic network aligning and stacking waveforms cancels noise and allows us to identify the rupture initiation and termination times accurately each stf is scaled to a reference moment of 1019 n m using the duration moment relation m01 3 in order to investigate depth dependence of the stfs we bin them into three depth ranges 100 350 km 350 550 km and deeper than 550 km based on physical properties of the mantle and the distribution of the data we infer a change in character near 550 km earthquakes deeper than about 550 km are shorter have simpler time functions and are more uniform in their characteristics further we find longer than expected scaled durations and greater complexity in the 350 550 km depth range the scaled durations of the 350 550 km earthquakes do not decrease with depth as 1 vs instead possessing a slightly longer average scaled duration than those 100 350 km deep we further categorize the earthquakes by subduction zone and find a similar depth dependence within individual zones finally the events that have achieved highest moment rate in the first 1 2 s after initiation tend to be deeper and or larger events the latter suggesting that dynamic rupture propagation affects the final size of the earthquake the depth dependence of duration and complexity suggests more complicated rupture histories between 350 and 550 km and a possible change in the physical mechanism of rupture at about 550 km

waveform effects of a metastable olivine tongue in subducting slabs we constructed velocity models of subducting slabs with a kinetically depressed olivine and spinel transition and examined the effect that such structures would have on teleseismic p waveforms using a full wave finite difference method these two dimensional calculations yielded waveforms at a range of distances in the downdip direction the slab models included a wedge shaped low velocity metastable olivine tongue moto to a depth of 670 km as well as a plausible thermal anomaly one model further included a 10 km thick fast layer on the surface of the slab the principal effect of moto is to produce grazing reflections at wide angles off the phase boundary generating a secondary arrival 0 to 4 seconds after the initial arrival depending on the take off angle the amplitude and timing of this feature vary with the lateral location of the seismic source within the slab cross section careful analysis of waveforms from earthquakes with depths near 400 km simple sources and adequate station coverage in appropriate geometries will be required to resolve whether moto is present

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

body wave inversion of the 1970 and 1963 south american large deep focus earthquakes a comprehensive set of teleseismic waveforms from two south american deep focus earthquakes of the predigital era the 1970 colombia mw 8 1 and 1963 peru bolivia mw 7 7 events are inverted for source mechanism seismic moment rupture history and centroid depth the p and sh wave inversion of the colombia event confirms previous work indicating that rupture occurred on a plane that dips steeply west rupture direction paralleled the trend of the wadati benioff zone we decompose the source into subevents based on a source time function which shows two major moment release pulses separated by 20 s the first subevent is located near the initiation point at a depth of 630 km the main moment release was located 70 km to the southeast and 20 km shallower rupture subsequently propagated farther southeast the source time function has an initial subevent accounting for 30 of the moment release of the entire event whereas the long period centroid moment tensor cmt analysis russakoff et al 1997 has the initial subevent yielding 50 the high angle nodal plane rotated 15 clockwise during the rupture explaining the large compensated linear vector dipole clvd component inferred from cmt solutions individual subevents have large clvd and compressive isotropic components a full moment tensor inversion of the colombia and 1994 bolivia events suggests that the initial subevents might contain a large non double couple ndc component for the 1963 peru bolivia event using p waves rupture propagated nnw for a distance of 70 km parallel to the high angle nodal plane and the trend of the wadati benioff zone the focal mechanism changed dramatically after the second subevent causing a very large ndc component both events together with the 1994 bolivia earthquake have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km s between subevents with 2 0 km s on average for the entire event low seismic efficiencies and rupture velocities support a highly dissipative temperature dependent rupture mechanism for large deep focus south american earthquakes compared with events in cold subducting slabs

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

deep and intermediate depth non double couple earthquakes interpretation of moment tensor inversions using various passbands of very broadband seismic data summary analysis of moment tensor inversions using various passbands of very broadband seismic data provides clear evidence that some non double couple moment tensors of intermediate depth and deep earthquakes result from the superposition of different double couple sources whose predominant principal axes are aligned with the predominant strain state within the subducting slabs the analyses are performed for three intermediate depth and deep earthquakes 1984 january 1 south of honshu 386 km 1985 april 23 luzon 181 km and 1987 may 7 northern sea of japan 417 km consistent non double couple moment tensors obtained using different sets of seismic waves in various low frequency bands suggest that these three significant non double couple components are not caused by unmodelled propagation errors of seismic waves in the inversion procedure because the various sets of seismic waves traverse very different ray paths for the south of honshu and the luzon events two major arrivals in the broadband p wave diplacement seismograms are observed with varying relative amplitudes or polarities from station to station the different double couple mechanisms which model the two phases combine to produce the significant non double couple moment tensors obtained at long periods the principal axes of the subevents closest in orientation to the predominant strain states within the slabs tend to be quite stable whereas the other two principal axes rotate between subevents this observation may explain the global nature of non double couple components in relation to the strain regime within the slab for the northern sea of japan event two major phases are observed in the p wave displacement waveforms but the variation in mechanism of two subevents that we model is not enough to explain the large non double couple component observed at long periods however since the non double couple component is exceptionally large compared with those for other deep earthquakes changes of focal mechanism with shorter delays than we can resolve appear to explain the overall radiation most simply

a hypothesis for the seismogenesis of a double seismic zone summary the seismogenesis of a double seismic zone in particular the lower layer of a double seismic zone has not been adequately explained in the literature on the basis of seismic data and geothermal structures along three well studied cross sections in the kuril kamchatka and japan subduction zones we investigate the temperature pressure conditions associated with seismogenic structures of the double seismic zones the corresponding tip loci seem to suggest that earthquakes observed in the lower layer and in the lower part below approximately 130 20 km of the top layer of a double seismic zone were caused by metastable phase transition a mechanism similar to that responsible for deep focus earthquakes only at lower temperature pressure conditions under this hypothesis the wedge shaped configuration of a double seismic zone is interpreted to represent the loci of the kinetic boundary of the phase transition according to theoretical experimental studies and the constraints imposed by our observations a likely candidate for such a phase transition is the metastable al rich enstatite decomposing into the assemblage of al poor enstatite plus garnet earthquakes in the upper part of the top layer were most probably due to conventional mechanisms such as dehydration of subducted materials and or facies change from basalt to eclogite that the top layer involves more than one seismogenic mechanism is also implied by the distinct behaviour of seismicity in the vicinity of 130 f 20 km because the presence of deviatoric stress is critical to the reaction rate of a metastable phase transition it is inferred that single seismic zones are also caused by the same mechanisms except that the implicit layer of a supposed double seismic zone is missing due to the insufficient amount of appropriate metastable minerals or to the lack of appropriate deviatoric stresses in the source region

acoustic emissions produced by anticrack faulting during the olivine spinel transformation anticrack faulting during the phase transformations of olivine to its denser polymorphs is a principal contender for the mechanism of deep focus earthquakes the original work on mg2geo4 established that initiation of faulting by this mechanism is not due to brittle fracture and subsequent experiments on olivine of mantle composition showed that it can operate at pressures of at least 14 gpa here we report acoustic emissions accompanying anticrack faulting in mg2geo4 thereby confirming that this mechanism involves a dynamic instability accompanied by elastic radiation

39ar 40ar ages and origin of excess 40ar in martian shergottites we report new 39ar 40ar measurements on 15 plagioclase pyroxene and or whole rock samples of 8 martian shergottites all age spectra suggest ages older than the meteorite formation ages as defined by sm nd and rb sr isochrons employing isochron plots only los angeles plagioclase and possibly northwest africa nwa 3171 plagioclase give ages in agreement with their formation ages isochrons for all shergottite samples reveal the presence of trapped martian 40ar 40arxs which exists in variable amounts in different lattice locations some 40arxs is uniformly distributed throughout the lattice resulting in a positive isochron intercept and other 40arxs occurs in association with k bearing minerals and increases the isochron slope these samples demonstrate situations where linear ar isochrons give false ages that are too old after subtracting 40ar that would accumulate by 40k decay since meteorite formation and small amounts of terrestrial 40ar all young age samples give similar 40arxs concentrations of 1 2 x 10 6 cm 3 g but a variation in k content by a factor of 80 previously reported nasa johnson space center data for zagami shergotty yamato y 000097 y 793605 and queen alexandra range que 94201 shergottites show similar concentrations of 40arxs to the new meteorite data reported here similar 40arxs in different minerals and meteorites cannot be explained as arising from martian atmosphere carried in strongly shocked phases such as melt veins we invoke the explanation given by bogard and park 2008 for zagami that this 40arxs in shergottites was acquired from the magma similarity in 40arxs among shergottites may reveal common magma sources and or similar magma generation and emplacement processes

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

directivity analysis of the deep bolivian earthquake of june 9 1994 the june 9 1994 deep bolivian earthquake m w 8 3 d 636 km occurred on a nearly horizontal rupture plane with an approximate area of 2000 km 2 the rupture plane is constrained by directivity analysis of the p wave displacement pulses recorded at local regional and teleseismic distances the fact that there are local recordings of the displacement provides a strong constraint on the vertical extent of faulting which is less than 10 km the moment release had at least four distinct episodes or subevents the best overall explanation for the subevents is rupture which initiates on the western margin of the fault plane propagates to the east and then continues bilaterally along a 350 azimuth with most of the moment release occurring to the north the apparent rupture velocity between subevents is very low approximately 1 2 km s

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

when slabs collide a tectonic assessment of deep earthquakes in the tonga vanuatu region an unusual group of more than 100 earthquakes is located 600 km beneath the north fiji basin in the southwest pacific ocean these earthquakes are attributed to seismicity within detached segments of the subducted australian plate one of these detached slab segments has collided with and now impinges on the subducted pacific plate at a depth of 500 km the region in the mantle where the two slab segments meet is also characterized by an unparalleled abundance of large magnitude mw 7 0 earthquakes for this depth the folded shape of the pacific slab beneath fiji as well as the abundance of earthquakes is interpreted to result from deformation and deformation enhanced phase transformations as the two slabs collide and settle on the 660 km discontinuity detachment of the slab segments is interpreted to have occurred at ca 5 ma but collision between the eastern segment and the west dipping pacific slab is interpreted to have occurred at ca 4 ma coincident with initial opening of the overriding lau basin

a simple explanation for the depth distribution of deep earthquakes in a model for the depth distribution of deep earthquakes i assume that deep earthquakes occur only above a critical deviatoric stress and that where the stress is achieved the seismicity rate is proportional to the strain rate the model seismicity rate increases sharply before terminating in agreement with the observed depth distribution since the assumptions are valid for most plausible seismic processes no large stresses or unusual earthquake mechanisms near the termination of seismicity are required

does maximum earthquake size depend on focal depth abstract while the gutenberg richter law gr with b value of 1 0 is an inexact description of the earthquake frequency magnitude distribution it does provide a convenient statistical basis for identifying when the largest earthquakes in a catalog are either anomalously large or anomalously small when the largest earthquakes are as predicted by the gr distribution it is inappropriate to infer that the largest historically known earthquake is the largest possible only when the largest observed earthquakes are significantly smaller than predicted can we infer that the largest possible earthquakes have occurred analysis of the global historical catalog demonstrates that at nearly all depths the largest deep and intermediate earthquakes have sizes close to those predicted by a gr distribution with b of 1 0 only between about 300 to 450 km and beneath 600 km are the largest known earthquakes somewhat larger than predicted however when geographically isolatable subgroups within catalogs are considered separately largest earthquakes that are either anomalously large or small are quite common often this is because individual regions have b values much different than 1 0 alternatively sometimes there appear to be physical constraints limiting the size of the largest possible earthquake as examples illustrating these assertions this article evaluates two subgroups deep focus earthquakes occurring beneath spain and intermediate depth earthquakes in the bucaramanga colombia nest

double seismic zones kinematics the kinematics of a double seismic zone of subducting lithosphere at an intermediate depth is studied a simple geometrical argument shows that the strain rate due to unbending of subducting lithosphere can be as large as 10 15 s 1 and large enough to account for the seismic strain rate in this depth range the geometrical unbending of subducting lithosphere in the mantle and the presence of a double seismic zone there can be therefore considered essentially equivalent phenomenon we model a double seismic zone required from this unbending in order to investigate what constraint the presence of a double seismic zone can place on the properties of the descending lithosphere the thickness of a double seismic zone is controlled by the thermal structure and the rheological properties of the subducting slab a preliminary result for the double seismic zone beneath tohoku japan indicates a possibility that the currently available flow law of dry olivine does not properly describe the double seismic zone or at least presents some inconsistency in our knowledge of the mechanical properties e g thermal structure rheology of oceanic lithospheres

topography of the transition zone seismic discontinuities phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities nominally at 410 520 and 660 km depth thermodynamic principles govern phase transformations making them sensitive to changes in the mantle s ambient conditions through the thermodynamic response changes in temperature or composition shift the transformation to a different pressure creating topography on a level discontinuity with this use as an exploratory tool for the mantle in mind we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities reviewing the evidence for and against the phase transition model we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

systematic relocation of early instrumental seismicity earthquakes in the international seismological summary for 1960 1963 abstract we have relocated 2556 events that were reported in the bulletins of the international seismological summary iss during the period 1960 1963 using the phase arrival time data listed in the bulletins parts of the data were already available in digital form and the rest we obtained by scanning the printed bulletins and applying an optical character recognition procedure earthquakes were then relocated with a teleseismic location method that uses an improved 1d global travel time earth model depth phases and corrections for ellipticity bounce point bathymetry topography and station elevation the aspherical velocity structure below stations was partially taken into account through the use of teleseismic station patch corrections the quality of the newly obtained locations and the improvements in data fit are similar to relocations of earthquakes in the early bulletins of the international seismological centre isc the most significant improvement of the new locations is in focal depth and a significant number of blatant mislocations such as earthquakes deeper than 100 km below midocean ridges have also been corrected the relocations presented here extend by 10 the time period for which homogeneous and reliable hypocenters are available on a global basis this is the first of a series of studies addressed towards relocating using the same methodology all the instrumentally recorded earthquakes listed in the bulletins of the iss 1918 1963

source retrieval for mantle earthquakes by iterative deconvolution of long period p waves abstract long period p waves of eighteen deep and intermediate depth earthquakes in a moment range 1 10 26 3 10 28 dyn cm are analysed by the method of kikuchi and fukao this issue to iteratively derive point dislocations on a plane the fault plane is defined as the plane of point dislocations that gives the best fit between the synthetic and observed waveforms for dip slip type earthquakes the fault plane is found to dip more steeply than the auxiliary plane in general point dislocations tend to cluster into one or a few subevents we characterize the largest subevent by its maximum moment rate m 0 m and its half duration from which the associated stress drop can be roughly estimated for earthquakes shallower than 400 km half duration is approximately constant 5 s so that stress drop increases from 1000 bar as m 0 m increases the value of for the great banda sea earthquake of 1963 h 100 km is exceptionally large 10 s for which 700 bar for earthquakes deeper than 400 km increases from 3 s to 9 s as m 0 m increases so that is approximately constant 700 bar stress drop so obtained approximates to an overall stress drop with a possible exception of the several largest earthquakes the distribution of point dislocations and the moment rate function indicate that rupture most often expands in a fan shaped fault area and that a large part of the rupture front ceases propagation in a coherent manner a sharp contrast in stress drop behavior across a depth of 400 km may be related to the olivine spinel transition within the downgoing slab of lithosphere

seismic body wave constraint on mechanisms of intermediate depth earthquakes 1 source parameters of six large earthquakes 7 1 mw 7 7 in the depth range between 112 and 277 km from five different subduction zones are derived from teleseismic body waves recorded by global seismic networks except for the 21 october 1995 mexican event which is consistent with the model of simple rupture on a planar surface the investigated earthquakes show complex rupture processes that can be explained by irregularities in the focal region as is the case of shallow events the inferred velocity of the moment release varies between 2 5 and 4 km s corresponding to 40 90 of the shear velocity at the focus four of the six earthquakes show stress drops in the range observed for shallow events 1 10 mpa ruptures extend mainly parallel to the strike of subducting slabs with dimensions reaching up to 50 km the extent of rupture perpendicular to slab dip of generally less than 15 km is comparatively small this suggests that the rupture during large intermediate depth earthquakes propagates predominantly parallel to the strike of the subducting plate and that the seismogenic zone perpendicular to the dip direction is limited to a narrow width the relatively small rupture size perpendicular to slab dip suggests that the rupture process does not extend into regions of significantly different temperatures the spatial extent of the investigated earthquakes is consistent with the orientation of trench parallel faults that are generally observed in near trench outer rise regions suggesting that these events may result from reactivation of such preexisting faults at intermediate depths through dehydration of hydrous minerals concentrated in these zones of weakness depths of most of the earthquakes are consistent with the dehydration embrittlement of serpentine the bolivian earthquake of 23 january 1997 seems to be too deep 277 km for such an explanation

changes in frequency size relationship from small to large earthquakes the constant b value observed in frequency magnitude distributions of earthquakes has been taken as an indication of self similarity at all magnitudes hence earthquake properties should scale uniformly in the same way from small to large earthquakes it has often been observed however that the seismic moment released in small earthquakes scales differently with rupture length than it does for large events1 2 where the crossover between small and large events is defined at a rupture dimension equal to the down dip width of the seismogenic zone a possible explanation for this contradiction is that frequency size distributions are biased by small earthquakes small events dominate most global earthquake catalogues because of the short time period covered another source of bias in size distributions is the saturation of earthquake magnitudes for large events here we correct biases in calculations of b values and present evidence for a change in b value in frequency size distributions we find that a break in self similarity from small to large earthquakes occurs at a point where the dimension of the event equals the down dip width of the seismogenic layer

seismic constraints on the morphology of deep slabs residual sphere images from deep earthquakes not only detect the presence of slab associated velocity anomalies but also lend insight into the flow and deformation of lithosphere subducted into the lower mantle we have compared travel times from deep events in the kuril and mariana arcs with the seismic velocity anomalies implied by kinematical models that thicken the slab perpendicular to its plane by reducing the vertical velocity of the flow with depth we assume that the details of the deformation whether the slab buckles imbricates fragments etc are averaged out along the ray paths and hence our models constrain the scale not the mode of slab thickening the deep event travel times are best fit by undeformed models but the ability of the residual sphere method to resolve slab thickness is limited by ray bending effects although the mariana times are consistent with advective thickening factors of 5 or more factors larger than 3 are ruled out by the kuril data for all models examined the data require that slab material extends to depths of 900 1000 km global tomographic models and regional studies which delineate high velocity anomalies in the lower mantle beneath zones of cenozoic subduction are consistent with our results as is recent work on pulse distortion by slab gradients comparison of observed and predicted rates of seismic moment release suggests that if substantial advective thickening does occur it is largely aseismic

the depth of the deepest historical earthquakes we usep ands times listed in the international seismological summary to relocate 23 historical earthquakes 1927 1963 reported as occurring at or below 670 km in all cases our relocated hypocenters are shallower than the starting depths furthermore all events converge to 691 km or less with a precision estimated at 10 km this study upholds the results of stark and frohlich who had usedpp p times of post wwssn earthquakes to constrain reliable hypocentral depths to no greater than 684 km in particular we reject rothe s claim that a 1963 event in the vicinity of new guinea occurred at a depth of more than 780 km

earthquake magnitude recent research and current trends abstract the magnitude recommendations adopted by the iaspei assembly at zurich in 1967 have had both a stabilizing and a stimulating effect on magnitude determinations and related research from 1967 onwards one magnitude research paper has appeared on the average almost every week thus making this parameter the most studied one in seismology new facilities and more accurate methods e g concerning instrumental equipment and interpretation techniques have made it possible to improve earlier achievements the application of magnitude scales has been extended in all respects e g with regard to epicentral distances focal depths wave types and wave periods magnitude frequency relations have become the most investigated equations within seismology observationally as well as theoretically they have wide applications e g for estimating the maximum magnitudes of future earthquakes an important item in earthquake prediction the magnitudes provide significant information on other source parameters such as wave energy fault length seismic moment relations between different types of magnitude yield valuable information on source properties for instance relations between magnitudes based on body and surface waves are used for efficient discrimination between earthquakes and underground explosions it is our purpose to review the magnitude development in the post zurich period 1967 1980 partly for geologists tectonophysicists and engineers who need an overview partly for seismologists who need an introduction to an overwhelmingly comprehensive literature

stopping phases in seismograms and the spatiotemporal extent of earthquakes on the basis of long period wwssn and some broadband seismograms from grafenberg we investigate the following earthquakes view this table p wave seismograms or true ground displacements show clearly the directivity effect of rupture propagation and the multiple event nature of these earthquakes fault plane solutions of individual source shocks and their relative localization by the master event technique yield fixpoints of rupture history the most striking feature is a strong stopping phase which is interpreted kinematically by abrupt termination of rupture expansion at the end of the fault we use the stopping phase to determine total source extent in space and time as well as the mean rupture velocity the results of this first stage of investigation are subsequently used for modeling the rupture according to the dislocation theory of shear faulting wave propagation through the earth s mantle is treated by ray theory this forward modeling approach yields seismic moment source dimensions mean dislocation and stress drop source complexities are related to unbroken fault gaps bending of the fault plane acceleration of rupture propagation in asperities and most impressive in its radiation effect to the sudden termination of rupture in front of a barrier source complexities are also discussed with respect to tectonic implications

seismic constraints on the water flux delivered to the deep earth by subduction for the first time large quantities of water are identified throughout the length of a subducting plate and an efficient pathway is defined for injecting water into the deep 600 km earth by the incorporation of water into the mantle of an oceanic plate as serpentine water is thought to facilitate plate tectonics but the quantities of water involved and thus its effect are poorly known taking advantage of an anomalous seismic arrival recorded in the tonga fiji subduction zone the presence volume water content and flux of serpentine and water associated with the subducting oceanic mantle at tonga is quantified at 2 10 8 tg ma h 2 o many times larger than previous inferences based on surface observations water fluxes of this magnitude into the deep earth will substantially increase the role of water in hypotheses of plate tectonic movements mantle plumes and flood basalt activity

heterogeneities of the instrumental seismicity catalog 1904 1980 for strong shallow earthquakes we analyze the world wide consistency of teleseismic reporting completeness of the seismicity record and homogeneity of magnitude determination for strong shallow earthquakes magnitude m 6 depth h 70 km in the period 1904 to 1980 for earthquakes with magnitude m 7 we used the catalog given by abe 1981 which lists the surface wave magnitude ms of every reported large event that occurred between 1904 and 1980 according to the original formulation of gutenberg 1945 under the postulate that the rate of earthquake occurrence for the entire world as a whole is consistent on a time scale of decades and that since the installation of the world wide standardized seismograph network in the early 1960s the earthquake catalog for strong m 6 shocks is complete and the seismicity rates are typical of all periods in the century we find that for instrumental related reasons the ms of large events prior to 1908 have been systemically overestimated by at least 0 5 magnitude unit and the ms of events in the period 1908 to 1948 have been consistently overestimated by 0 2 unit relative to the ms assigned to shocks occurring after 1948 when these corrections are taken into account the catalog of events with ms 7 0 is shown to be largely complete i e nearly all the large shallow shocks which occurred in the earth are listed for most of the world since early in the century for earthquakes as the magnitude 6 level we used the international seismological center isc tape and regional catalogues of earthquakes this file compiles the earthquake data given by gutenberg and richter 1954 for the first half of the century and data from various international agencies after 1945 we find a highly incomplete seismicity catalog at this magnitude level for the first half of the century mainly due to numerous increases and decreased in the earthquake detection and reporting capabilities a more complete but extremely inhomogeneous catalog for the period 1950 to 1963 due to the use of different formulations and criteria to calculate and report the parameter magnitude by the various seismological agencies and a largely complete and homogeneous in ms record since 1964

complements to magnitudes of large shallow earthquakes from 1904 to 1980 abstract in the previous paper magnitudes of large earthquakes of the world were determined on a very uniform basis and catalogued in order to arrive at a more homogeneous catalogue for a study of global seismicity some complements are presented the major complements are as follows 1 the m s values for the period 1904 1909 in the previous paper were overestimated in the extreme case by 0 5 magnitude units 2 the m b values for the period 1904 1908 in the previous paper were also overestimated 3 the magnitudes of deep and intermediate shocks which are listed in seismicity of the earth by gutenberg and richter are found to be approximated by m b m b 7 4 4 the catalogues of m b for shallow intermediate and deep shocks are continued for the period up to 1980

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

origin and significance of deep earthquake clusters surrounding a pronounced seismic gap in northeast china abstract from analysis of earthquakes in the global centroid moment tensor gcmt catalog we identify an intermediate depth 250 350 km seismic gap having dimensions of 300 km 500 km within the subducting northwestern pacific plate npp and surrounded by several groups of isolated earthquakes near northeast china normally groups of unusual deep earthquakes may represent a detached slab segment as in tonga vanuatu and a deep seismic gap maybe either be the aseismic continuation of a subducting slab or a possible gap in the subducting plate we evaluate the characteristics of deep earthquakes in the npp and focus on three isolated clusters particularly one directly beneath the seismic gap and northeast china this sub gap cluster has a peak in the number of quakes near depth 580 km unlike seismicity in the npp as a whole its b value 0 60 is significantly smaller than b values in two other nearby clusters 0 81 and 0 86 in the npp focal mechanisms in this sub gap cluster also show a variation with depth and projections of their t p and b axes differ from what is observed for the other isolated clusters we speculate why these sub gap earthquakes are isolated and suggest two possible models explaining the presence of the gap and the characteristics of the underlying cluster beneath northeast china in the first model the seismic gap is attributable to aseismic deformation caused by abnormally high temperatures in the subducting lithosphere and the characteristics of the sub gap clusters and its related slab structures are attributed to slab buckling caused by resistance to subduction in the second model the seismic gap represents absent subducting lithosphere and the missing material has migrated to the bottom of subducting slab although we favor the second model both models are consistent with our knowledge of deep earthquakes and the results of recent studies near northeast china

thermally induced phase changes lateral heterogeneity of the mantle continental roots and deep slab anomalies pressure induced solid solid phase changes are responsible for most of the increase of density and seismic velocity with depth in the upper mantle lateral variations in temperature cause a similar effect abrupt changes of density and seismic velocity due to phase changes are superposed on smaller changes associated with thermal expansion temperature induced isobaric phase changes are as important in explaining various recent geophysical data as are the more familar pressure induced phase changes in cold slabs the equilibrium mineral assemblage contains high density high velocity phases which are not stable in hotter mantle in particular the ilmenite form of mgsio 3 and the spinel form of mg 2sio 4 have broad stability fields in cold mantle which increase the density and velocity of deep slabs to values in excess of those which have been used in geoid and seismic travel time modeling recent arguments for slab penetration into the lower mantle and whole mantle convection are based on thermal models of the slab which ignore the large density and seismic velocity anomalies associated with temperature induced phase changes when these effects are taken into account the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and layered models of mantle convection the seismicity cutoff and evidence for slab thickening at 670 km also favor this style of convection mantle seismic velocities between 200 and 400 km depth in tectonic and young oceanic regions are lower than in shield regions and this is due to the presence of a melt phase and lower velocity high temperature phase assemblages deep 200 km long lived continental roots differing in chemistry from normal mantle are not required when isobaric phase changes are taken into account high velocity subshield mantle is closer to normal subsolidus mantle than is suboceanic mantle which is affected by the presence of high temperature phase assemblages the whole mantle convection thick continental root and deep slab penetration hypotheses are not supported by seismic and geoid data when isobaric phase changes are included in the analysis phase changes are more effective in changing density and seismic velocity than are lateral variations in temperature and composition the lack of correlation of the geoid with ridges shields heat flow and upper mantle velocity variations suggests a low geoid sensitivity to the upper mantle consistent with layered convection

application of the relative amplitude moment tensor program to three intermediate depth iaspei earthquakes abstract a new method of determining moment tensor components from teleseismic relative amplitude observations is applied to three of the earthquakes selected for discussion at the 1985 general assembly of iaspei the method places constraints on the source mechanisms of earthquakes using simple measurements taken from teleseismic records and reveals both the source type and its orientation explicitly well constrained solutions are obtained for all three earthquakes and the results illustrate the ability of the method to discriminate between different source types and to determine source orientation for events where there is little separation between p and its surface reflected phases on long period seismograms

frequency distribution and quantification of deep earthquakes the frequency distribution of deep earthquakes d 350 km is analyzed on a regional scale using magnitude data from 4660 events reported in the international seismological center bulletin 1964 1982 and 391 centroid moment tensor solutions 1977 1987 we use the moment magnitude mw to define b values comparable with studies of shallow seismicity scaling laws relating mw mb and mo are derived mw 1 6mb 3 2 mb log mo 4 3 2 4 which compare very favorably with published laws uncertainties and biases of the mb and mo data sets are considered the slope of the frequency distribution is evaluated over a restricted magnitude range the results are very robust and independent from the magnitude range used from the mb magnitude data we obtain regional b values in the range 0 4 b 1 22 with a worldwide average value b 0 87 these results are confirmed by the analysis of the seismic moment data the steepest frequency distribution is observed in the tonga islands region b 1 22 although 60 of the worldwide deep seismicity occurs here only few events since 1977 had seismic moment exceeding 1019 n m the high fractal dimension d 2 4 implies a volume partially filled with fault planes in agreement with the observations on the seismic morphology of the region at the other extreme south america is characterized by the largest recorded deep events and an almost complete absence of small ones b 0 4

deep earthquakes and the thermal assimilation of subducting lithosphere two problematic aspects of deep seismic zones are investigated the maximum depth about 700 km of seismic activity and the relative maximum in seismic energy release in the 500 700 km depth range it is shown that both features can be accounted for by a model based on thermal assimilation this implies that no barrier is required at a depth of 650 700 km to explain the observations

passive and active seismological study of bending related faulting and mantle serpentinization at the middle america trench water transported within the subducting oceanic lithosphere into the earth s interior affects a wealth of subduction zone processes including intraslab earthquakes and arc magmatism in recent years growing evidence suggests that much of the hydration of oceanic plates occurs at the trench ocean slope right before subduction here normal faults are created while the rigid lithosphere bends into the trench offshore of middle america multi channel seismic reflection imaging suggests that bending related faults cut into the uppermost mantle providing a mechanism for hydration and transformation of mantle peridotites into serpentinites seismic wide angle reflection and refraction data were collected coincident with one of the seismic profiles where the faults have been imaged travel time inversion provides evidence that both crustal and uppermost mantle velocities are reduced with respect to the velocity structure found in mature oceanic crust away from deep sea trenches if mantle velocity reduction is solely produced by hydration velocities indicate 10 15 of serpentinization in the uppermost 3 km of the mantle where seismic data provide enough resolution a small network of ocean bottom hydrophones deployed for about a month detected 3 local micro earthquakes per day earthquake epicentres align with fault scarps at the seafloor and continuous earthquake activity might be an important process to facilitate the percolation of seawater into the upper mantle

the present status of reservoir induced seismicity investigations with special emphasis on koyna earthquakes abstract the status of reservoir induced seismicity ris has been reviewed periodically rothe 1968 1973 gupta and rastogi 1976 simpson 1976 packer et al 1979 in the present paper the significant work carried out during the last three years on ris is reviewed an earthquake of magnitude 5 1 2 occurred on november 14 1981 in the vicinity of aswan lake egypt 17 years after the filling started in 1964 this event occurred 4 days after the seasonal maximum in the reservoir water level and was followed by a long sequence of aftershocks another event of magnitude 4 1 2 occurred in the vicinity of aswan lake on august 20 1982 results of preliminary investigations indicate that this seismic activity is reservoir induced recent analyses of induced seismic events at nurek reservoir u s s r show that the second stage of filling during august to december 1976 increasing the maximum depth from 120 m to 200 m was accompanied by an intense burst of shallow seismic activity an outward migration from the centre of the reservoir possibly associated with diffusion of pore pressure is revealed by the temporal distribution of earthquake foci a variety of investigations including the in situ measurement of tectonic stress pore pressure permeability distribution of faults etc in addition to monitoring seismicity have been undertaken in the vicinity of the monticello reservoir south carolina the largest reservoir induced earthquake is predicted not to exceed magnitude 5 the koyna reservoir india continues to be the most outstanding example of ris three earthquakes of magnitude 5 occurred in september 1980 earthquakes of magnitude 4 occur frequently in the vicinity of koyna the latest being on february 5 1983 events that occurred during the period 1967 1973 have been relocated using better procedures and are found to be much shallower and the epicentres less diffused location of 12 earthquakes of ms 4 0 their foreshocks and aftershocks that occurred during 1973 1976 composite focal mechanism solutions and related studies are consistent with the delineation of a n s trending fault through the reservoir area in a couple of interesting studies it has been demonstrated that earthquakes of magnitude 5 0 in the koyna region are usually preceded by several magnitude 4 earthquakes in the preceding fortnight also a rate of loading of koyna reservoir of at least 40 ft week appears to be a necessary although not sufficient condition for the occurrence of magnitude 5 earthquakes smooth filling emptying appears to be the key to reduce the hazard of ris a map and a table of the reported cases of reservoir induced changes in seismicity through 1982 have been compiled

mechanics of non critical fold thrust belts based on finite element models abstract the mechanics of fold thrust belts and accretionary wedges is investigated using a two dimensional plane strain elastic plastic cohesive mohr coulomb mechanical model solved with the finite element method results show that when a layer with an initially non critical geometry is compressed from the rear it does not form a wedge that is at failure throughout as assumed in critical wedge theory rather the wedge consists of narrow plastic shear zones that propagate sequentially outward with time loading rocks ahead while unloading rocks behind not only are stress states within the wedge not everywhere at failure but principal stress orientations vary strongly in time and space particularly across shear zones near the basal detachment and in the hanging wall of active structures where local surface extension may be observed the reason the investigated wedges are not stressed to compressive failure throughout is related to strength reduction associated with strain localisation that enables material outside shear zones to unload and return to an elastic stress state this mechanism is intrinsic to elastic plastic materials and occurs regardless of any material degradation such as loss of cohesion even though the stress state of the investigated wedges is generally non critical the overall geometry may still be consistent with cohesionless critical wedge theory since the local surface slope is created when a particular part of the wedge is at a limit state prowedge tapers display non self similar growth through time but eventually evolve to the minimum critical taper retrowedges on the other hand may get caught within this initial transient state and thus may have tapers anywhere between the minimum and maximum critical taper however if the basal detachment is such that lateral propagation is not kinematically inhibited retrowedges are shown to also eventually evolve towards minimum critical tapers resulting in a symmetrical doubly vergent orogen because all of these wedges have non critical stress states both pro and retrowedges are predicted to display stable non equilibrium taper angles in the presence of external perturbations for example related to erosion or sediment deposition