the 9 june 94 bolivian deep earthquake an exceptional event in an extraordinary subduction zone we investigate the physical setting of the bolivian shock based on the history of the subducting nazca plate intraslab seismicity deep seismic moment release and seismic tomography south america has two broad regions of reverse arc curvature subduction constrained to this unique geometry produces slab kinking contortions that may cause unusual slab thickening as they sink to the bottom of the transition zone and encounter resistance to penetration into the lower mantle such contortions are observed at intermediate depths upstream from the slab source regions of both the great 1970 colombian and 1994 bolivian events thickening helps explain how the nazca slab accommodates large seismic source dimensions at depths of 625 650 km

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

volcanic plume height correlated with magma pressure change at grimsvotn volcano iceland the 2011 eruption of a 20 km high volcanic plume from grimsvotn volcano iceland led to the closure of northern european airspace geodetic measurements from the volcano reveal a correlation between plume height surface deformation and magma chamber pressure with a delay of an hour implying that volcanic plume behaviour can be predicted before eruption onset

geochemistry of evolved magmas and their relationship to subduction unrelated mafic volcanism at the volcanic front of the central mexican volcanic belt abstract this study reports new geochemical and radiogenic isotope data on hy normative pleistocene to recent 87 sr 86 sr 0 70370 0 70469 143 nd 144 nd 0 51278 0 51289 206 pb 204 pb 18 64 18 72 207 pb 204 pb 15 59 15 62 and 208 pb 204 pb 38 39 38 52 these isotopic ratios are generally similar to those for the associated mafic rocks although some differences do exist between them particularly for the 87 sr 86 sr ratios that are somewhat higher and 143 nd 144 nd that are slightly lower for the evolved rocks the available geochemical evidence does not support the generation of the scn evolved magmas by simple fractional crystallization fc or even by assimilation coupled with fractional crystallization afc of such mafic magmas nor by direct slab melting or indirect fluid transport to the mantle participation of the subducted cocos plate the viable mechanism seems to be that the most evolved andesitic and dacitic magmas in the scn were produced by partial melting of a heterogeneous mafic granulite source in the lower crust the magmas of intermediate compositions may reflect magma mixing processes between the most evolved andesitic and dacitic magmas generated in the lower crust and the mantle derived mafic magmas basalt and basaltic trachy andesite at the volcanic front of the central part of the mvb these results have direct bearing to other volcanic areas with complex tectonic setting

on b be ratios in the mexican volcanic belt boron and beryllium concentrations were measured in a diverse suite of well characterized rocks from the mexican volcanic belt mvb low b and high be result in relatively low b be ratios in the mvb compared to other arcs nevertheless b systematics resemble those of other arcs and provide insights into mantle processes in the mvb b enrichment depends first on magma type and second on edifice type and location b be values are highest 5 15 in andesites and dacites erupted from calcalkaline strato volcanoes located along the volcanic front such as volcan colima and v san juan rocks from strato volcanoes located behind the volcanic front generally have lower values 1 5 b be values are also elevated in differentiated members of rock suites that show evidence for significant crustal assimilation in the westernmost mvb west of the michoacan guanajuato volcanic field mgvf cindercone ejecta including basalts lamprophyres and basanites contain low b be values 5 the lamprophyres and basanites have very low b be despite high ba ce and other common measures of subduction signature in the mgvf where cinder cones occur exclusively b be values in primitive calc alkaline basalts are distinctly higher than those from alkaline basalts 3 8 vs 1 3 indicating that high b be is a mantle derived feature and not an artifact of crustal assimilation comparison among various elemental ratios indicates that cs and u show enrichment patterns similar to b all are enriched in calc alkaline rocks but not in lamprophyres or basanites in contrast ba k and sr are enriched in both calc alkaline rocks and the lamprophyres and basanites multi stage processes and differing melting mechanisms are inferred to explain the variable characteristics of mvb volcanic rocks first slab derived fluids rich in fluid mobile elements including b and ba infiltrate the mantle wedge these fluids cause fluid fluxed melting that produces calc alkaline magmas enriched in all fluid mobile elements these lavas erupt from large volcanic front strato volcanoes the slab derived fluids also metasomatize portions of the mantle wedge producing phlogopite and or amphibole these phases have high partition coefficients for ba sr k but may have low partition coefficients for b cs u accordingly subduction zone metasomatism produces a mantle wedge enriched in ba sr k but not necessarily in b cs u decompression melting of this type of metasomatized mantle will consume the hydrous phases and produce magmas such as lamprophyres or basanites with high ba ce sr la and k la but low b be cs la and u la this interpretation implies two types of subduction zone signatures one involving enriched ba sr k elements that have longer residence times in the mantle wedge and another involving enriched b cs u which all partition so strongly into fluids or melts that they have short residence times in the mantle and are only enriched in magmas generated by fluid fluxed melting or that have assimilated crustal material assimilation of granites and crustal rocks can also enrich differentiated lavas in b

tectonic implications of serpentinite weakening summary the observation that serpentinite becomes weak and brittle at its dehydration temperature provides an experimental basis for the hypothesis that mantle earthquakes are caused by shear fracturing where interstitial fluid pressure effectively reduces the frictional resistance to a low or negligible value if convection currents in the mantle turn down at the foredeeps of the pacific island arcs dehydration of an oceanic crust of serpentinite and downward convection of the dehydration products beneath the island arc would give rise to the anomalously low shallow mantle velocities and heat flows in the region between the trenches and island arcs seismic activity would be confined to the zone along which the weak and brittle dehydration products move downward against the immobile continental mantle and would be concentrated at the leading edge of the focal zone due to release of stored elastic strain upon the initiation of weakening pore fluid pressures giving rise to faulting in the deepest earthquakes are considered to be due to interstitial magma

application of the cmt algorithm to analog recordings of deep earthquakes abstract the distribution and characteristics of deep earthquakes provide important information on both the large scale aspects of mantle convection as well as on the small scale mechanics of apparent shear failure within descending lithospheric plates at large confining pressures neither of these processes is well understood but the detailed knowledge of the spatial and temporal distribution of earthquakes as well as the focal geometries scalar moments and details of the earthquake rupture process provides constraints which can be used to differentiate between competing models of mantle circulation and theories of deep seismic rupture with the objective of expanding and improving the database of source parameters for deep earthquakes we evaluate the feasibility of applying the harvard centroid moment tensor cmt algorithm to earlier pre 1977 earthquakes recorded on seismographs with analog recording abundant records from the world wide standardized seismograph network wwssn are available for events after 1962 and the data from these high quality stations with a known standard response are easily digitized and subjected to analysis by the cmt algorithm for earlier events no standardized network of seismometers is available using a modem deep earthquake as a test case we show that reliable single station moment tensor solutions can be obtained from the press ewing instrument at pasadena we apply the single station technique to three historical earthquakes recorded on different types of electromagnetic and mechanical seismographs the 1957 java earthquake press ewing the 1922 peru earthquake wiechert and the 1911 peru brazil earthquake galitzin robust results can be achieved from these data provided the instruments are well calibrated and an analysis of selected earlier deep earthquakes may quickly double or triple the time span of the database of deep earthquake source parameters

the deep earthquakes of 1921 1922 in northern peru abstract we present a detailed analysis of the 1921 and 1922 deep events in northern peru which we identify as the only reliably located earthquakes in the vicinity of the great 1970 deep colombian shock all other reported seismicity in a three dimensional radius of 550 km centered on the 1970 hypocenter is erroneous and can be relocated to other seismic provinces the 1921 1922 earthquakes relocate approximately 230 km to the south of the 1970 event at depths of 630 km and 660 km respectively a combination of centroid moment tensor inversion and forward modeling using ray theory and reflectivity synthetics yields focal mechanisms indicating down dip compression and seismic moments of 1 2 1027 dyn cm and 6 1027 dyn cm respectively the 1921 1922 and 1970 events take place on a piece of subducted farallon plate aged approximately 55 ma i e anomalously young to support deep seismicity we suggest that this unusual seismic activity may be due to a local heterogeneity on the slab whose nature however remains unknown

calibration of the hglp seismograph network and centroid moment tensor analysis of significant earthquakes of 1976 abstract a methodology is developed for determining accurate instrument response functions for seismographs of the high gain long period hglp network recordings of daily transient calibration pulses provide a detailed history of the time varying responses of the hglp stations when the instrument displacement transfer function is available knowledge of the seismometer mass calibration coil constant and calibration current allows for the prediction of the shape and amplitude of the calibration signal by simple integration of the transfer function in theory the inverse of this statement is also true but in practice noise in the recordings contaminates the instrument response function recovered through direct differentiation of the calibration pulse the unknown transfer function of the seismograph system is instead first described by the product of the nominal responses of its mechanical and electromagnetic components specific values of the characteristic parameters for each seismograph component are subsequently determined by iterative least squares minimization of the misfit between observed and model transient calibration pulses by application of this method to daily calibration pulses recorded on the ten stations of the hglp network refined instrument response functions for 1976 and part of 1975 are determined the results agree well with single frequency amplitude calibration data recovered from station maintenance records and produce synthetic waveforms which are consistent with data from other well calibrated stations we use the recalibrated hglp data together with long period data from early international deployment of accelerometers ida and seismic research observatory sro digital stations to determine 108 centroid moment tensor cmt solutions for moderate and large earthquakes of 1976 the new cmt results for several major earthquakes including the tangshan guatemala and friuli events are compared with those of previous studies

the depth dependence of earthquake duration and implications for rupture mechanisms the duration of rupture is a fundamental characteristic of earthquakes and is important for understanding the mechanics of faulting1 2 the complexity of the seismic source and the incoherence of the high frequency seismic wavefield often inhibit the identification location and timing of features in the later part of earthquake rupture here we sum many teleseismic records from regional seismic arrays producing an unusually clear depiction of the earthquake source at short periods by suppressing background noise and coda generated near the receivers the ending as well as the beginning of rupture is clearly identifiable for most earthquakes examined measurements of 130 large earthquakes show that near 100 km depth rupture duration averages 11 s when scaled to a moment of 1026 dyn cm this decreases to 5 5 s at 650 km depth models of faulting suggest that duration should be inversely proportional to the shear wave velocity and the cube root of stress drop thus to explain the observed twofold decrease in duration with depth stress drops would have to increase by a factor of four as shear velocity increases with depth by only about 20 however observed stress drops show no strong trend with depth3 4 suggesting that the faulting process changes with depth

tectonics along the trans mexican volcanic belt according to palaeomagnetic data abstract subduction of the cocos plate beneath the southern mexico continental margin takes place at the middle america trench volcanism of calc alkaline character occurs along the e w trending trans mexican volcanic belt tmvb which includes several active stratovolcanoes monogenetic cinder cone fields and silicic centres however a direct association between the volcanism and plate subduction has been questioned for a number of reasons including the oblique orientation of the tmvb with respect to the trench most palaeomagnetic directions for the cretaceous to quaternary within or immediately to the south of the tmvb appear discordant the declinations are rotated anticlockwise relative to expected declinations this angular divergence may be explained by rotations of crustal blocks by different degrees in response to large scale left lateral shear across central mexico the fault system may have acted as an efficient structural control for the rise of magmas which may explain some of the peculiarities of the tmvb

preliminary determination of focal mechanisms from the inversion of spectral amplitudes of mantle waves abstract we present a method for the preliminary determination of focal mechanisms pdfm based on the inversion of deviatoric moment tensor components from depleted datasets of mantle wave spectral amplitudes the latter are found to be largely insensitive to errors in epicentral estimates which can hamper the inversion of the full complex spectrum under real time operational conditions robust solutions can be obtained from rayleigh wave spectra at as few as three stations and from a combination of rayleigh and love waves at only two stations provided they sample 60 in azimuth the systematic application of this method to a dataset of 43 large earthquakes during 1996 1999 yielded mechanisms rotated an average of only 17 from the published harvard cmt a figure comparing favorably with the case of other catalogues such as the usgs moment tensor solutions

resolution experiments for nw pacific subduction zone tomography we investigate the resolving power of isc neic p travel time data in tomographic inversions for the geometry of the subduction zones in the nw pacific from thermal models for the kurile japan izu bonin mariana and ryukyu slabs we generate three dimensional synthetic velocity anomalies for subducting slabs which are then projected onto a cell model for the uppermost 1400 km of the mantle using these synthetic models we compute synthetic delay times for ray paths corresponding to the source and receiver locations used for the actual data add gaussian noise invert the synthetic data and compare the resulting velocity structure to the initial synthetic models this comparison is illustrated for sections through the kuriles and the mariana arcs a variety of resolution artifacts are observed which in many cases resemble features visible in the tomographic results obtained from inverting the actual isc neic data

seismicity shear failure and modes of deformation in deep subduction zones abstract planes of shear failure in deep seismic zones are identified as being parallel to focal mechanism nodal planes of deep focus earthquakes we relocate deep seismicity reported in the international seismological centre isc catalogue for earthquakes deeper than 400 km in the honshu bonin marianas java banda and south america subduction zones using the joint hypocentral determination method recent events with harvard centroid moment tensor cmt solutions and nearby seismicity are viewed from the direction of the cmt null axis comparing the two orthogonal nodal planes with the seismicity pattern planar features of seismicity parallel to cmt nodal planes are found in each deep seismic zone and are identified as planes of shear failure the sense of displacement on these planes is one of resistance to deeper penetration honshu and bonin show imbrication and horizontal displacement of the deepest part of the seismogenic slab while marianas and brazil show conjugate shear failure banda and argentina show patterns of vertical and horizontal shear planes for the western pacific slabs we find significant lateral displacements towards the south of the deepest part of the subduction flow tonga has been analyzed previously showing the same pattern as identified here in all benioff zones planes of shear failure are observed which act to shorten the slabs and displace matter horizontally normal to the slab moreover differential faulting acts to rotate the slab of subducted lithosphere to a more horizontal posture indicating that slabs experience resistance to penetration of the 670 km discontinuity

re evaluation of the large deep earthquake of january 21 1906 abstract the large deep earthquake of january 21 1906 is re evaluated using old seismogram data and updated analysis techniques from the p and pp p time data the hypocentre parameters are determined as follows origin time 13h 49min 35s latitude 33 8 n longitude 137 5 e depth 340 km the body wave magnitude m b is re evaluated from the amplitude and periods of p pp and s waves the average value of 7 4 is obtained this value is the smallest among any values assigned previously to this shock and it is denied that the earthquake is the world s largest deep shock in this century the focal mechanism is estimated from the p wave first motions and amplitude distribution of p and s waves synthetic body waves are used to constrain the mechanism and to determine the seismic moment the mechanism solution suggests the down dip compression typical of this region a seismic moment of 1 5 10 27 dyn cm is obtained this value and the re evaluated value of m b are consistent with the moment b relation obtained for other deep earthquakes

mapping of p wave slab anomalies beneath the tonga kermadec and new hebrides arcs abstract p wave velocity anomalies of several per cent fast relative to the jeffreys bullen jb model are seen around subduction zones under the tonga kermadec and new hebrides trenches from inversion of international seismological centre isc travel time residuals from intermediate and deep focus earthquakes mantle heterogeneities outside the model are corrected using previous velocity models ray paths inside the model region are traced allowing two dimensional velocity variations the rays of similar source and receiver locations are combined into summary rays and the hypocenters are relocated as part of the inversion iterations the resulting images are interpreted in conjunction with resolution and error estimations the slab like anomalies are generally continuous in places of high seismicity and decrease in amplitude with depth most coherent slab anomalies are surrounded by large slow patches between the surface and 350 km depth the fast slab beneath the tonga arc appears to end at 550 km depth before the seismicity ends but there is a fast band at 750 1000 km depths below the deepest tongan earthquakes the fast slab like anomaly beneath the kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle and it occurs aseismically near new zealand down to 500 550 km depth interpretations in the lower mantle in many places such as kermadec and new hebrides arcs however are hampered by poor resolution

interslab earthquakes and phase changes in subducting lithosphere at 0033 ut on 9 june 1994 a great deep earthquake occurred 635 km beneath the amazonian rain forest of northern bolivia the deep rupture of this record setting magnitude 8 3 shock lasted about a minute harvard university centroid moment tensor catalogue the seismic strain release of this single event as indicated by its seismic moment was greater than the total strain release for all deep earthquakes in the harvard cmt catalogue during the previous 17 years of its coverage of global seismicity incredibly ground motion was felt as far north as seattle washington and toronto canada evidently it occurred within the subducting nazca plate that is descending beneath south america this earthquake reminds us of the sometimes grand scale of deep seismic failure and of the need to consider more closely the physical processes that accompany slab descent and in particular how such earthquakes may reflect the inner workings of slabs

single station cmt application to the michoacan mexico earthquake of september 19 1985 observations of three dimensional ground motion at a single point on the surface of the earth following an earthquake can be sufficient to determine the zeroth order moment tensor of the event we adapt the centroid moment tensor cmt method dziewonski et al 1981 of analysis to single station data and apply it to the september 19 1985 michoacan earthquake the obtained mechanism indicates faulting on a shallow plane consistent with the ongoing subduction of the cocos plate the scalar moment is 1 3 1028 dyne cm a direct deconvolution of the observed p wave train indicates that the moment release occurred in two subevents each of 20 seconds duration with a peak to peak separation in time of 25 seconds

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

a reassessment of the deep fiji earthquake of 26 may 1932 abstract the 1932 deep fiji earthquake is studied from the point of view of comparing it to the recent large events in the transition zone spectral analysis and body wave modeling suggest a moment of 3 4 10 27 dyn cm within a multiplicative or divisive factor of 1 5 which makes it comparable to but not significantly larger than the great 1994 earthquake 770 km to the north its seismic moment remains well below that of the largest deep shocks 1994 in bolivia or 1970 in colombia relocation efforts show that the earthquake took place on the front edge of the wadati benioff zone in an area where the latter features a complex geometry involving significant warping the mechanism of the earthquake involves down dip compression and can be readily explained in terms of the prevailing large scale stresses in the slabs

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

centroid moment tensor solutions for deep earthquakes predating the digital era discussion and inferences abstract we present a discussion of 139 new centroid moment tensor solutions for deep earthquakes covering the wwssn 1962 1976 and earlier years obtained by applying the standard harvard inversion algorithm to data sets of seismograms digitized from analog records our solutions fill several gaps in seismicity reflecting undersampling by the harvard catalogue while most focal mechanisms previously published for wwssn era earthquakes are in good agreement with our inverted solutions focal geometries for older events appear unreliable we confirm that the region of most intense moment release is around 650 km and that a minimum exists at 500 km however the local maximum around 450 km may be less marked than previously suggested the bonin marianas subduction zone exhibits a different pattern with an overall maximum of seismic activity at 420 km all other subduction zones demonstrate the potential for seismicity at least at the 1027 dyn cm level finally while non double couple components are found systematically in our enhanced data set they correlate neither with depth nor with earthquake size suggesting that their presence does not necessarily relate to the exiguity of the space available for rupture at the tip of subduction zones

faulting associated with the olivine to spinel transformation in mg2geo4 and its implications for deep focus earthquakes samples of metastable mg2geo4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel the stable phase just becomes possible at lower temperatures where the transformation is inhibited metastable olivine is ductile and strong while at higher temperatures where the transformation happens very quickly samples are ductile and weak microfaults that have just been created and have slipped very little contain only very fine grained spinel those that have slipped more contain a mixture of fine grained spinel and larger broken olivine fragments a new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine grained lens shaped bodies with a strong preferred orientation normal to maximum compression the correlation between the onset of transformation the new microstructure and the mechanical instability suggests a cause and effect relationship the grain size of the spinel within the microfaults suggests that these zones have superplastic properties in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed analogous conditions exist in down going slabs of lithosphere in the mantle and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep focus earthquakes

bibliographical search for reliable seismic moments of large earthquakes during 1900 1979 to compute mw in the isc gem global instrumental reference earthquake catalogue abstract moment magnitude m w determinations from the online gcmt catalogue of seismic moment tensor solutions gcmt catalog 2011 have provided the bulk of m w values in the isc gem global instrumental reference earthquake catalogue 1900 2009 for almost all moderate to large earthquakes occurring after 1975 this paper describes an effort to determine m w of large earthquakes that occurred prior to the start of the digital seismograph era based on credible assessments of thousands of seismic moment m 0 values published in the scientific literature by hundreds of individual authors m w computed from the published m 0 values for a time period more than twice that of the digital era are preferable to proxy m w values especially for earthquakes with m w greater than about 8 5 for which m s is known to be underestimated or saturated after examining 1 123 papers we compile a database of seismic moments and related information for 1 003 earthquakes with published m 0 values of which 967 were included in the isc gem catalogue the remaining 36 earthquakes were not included in the catalogue due to difficulties in their relocation because of inadequate arrival time information however 5 of these earthquakes with bibliographic m 0 and thus m w are included in the catalogue s appendix a search for reliable seismic moments was not successful for earthquakes prior to 1904 for each of the 967 earthquakes a preferred seismic moment value if there is more than one was selected and its uncertainty was estimated according to the data and method used we used the iaspei formula iaspei 2005 to compute direct moment magnitudes m w m 0 based on the seismic moments m 0 and assigned their errors based on the uncertainties of m 0 from 1900 to 1979 there are 129 great or near great earthquakes m w 7 75 the bibliographic search provided direct m w values for 86 of these events or 67 the gcmt catalog provided direct m w values for 8 events or 6 and the remaining 35 or 27 earthquakes have empirically determined proxy m w estimates an electronic supplementary file is included with this paper in order to provide our m 0 m w catalogue of earthquakes 1904 1978 from the published literature and a reference list of the 1 123 papers that we examined

element fluxes associated with subduction related magmatism destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element ree ratios most island arc suites have low ce yb and remarkably restricted isotope ratios of 87 sr 86 sr 0 7033 143 nd 144 nd 0 51302 206 pb 204 pb 18 76 207 pb 204 pb 15 57 and 208 pb 204 pb 38 4 however they also have rb sr 0 03 th u 2 2 and ce yb 8 5 ratios which are significantly less than accepted estimates for the bulk continental crust the high ce yb suites have higher incompatible element contents more restricted heavy ree and much more variable isotope ratios such rocks are found in the aeolian islands grenada indonesia and philippines and their isotope and trace element features have been attributed both to contributions from subducted sediment and or old trace element enriched material in the mantle wedge it is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid ocean ridge basalts since these may contain ca 80 of the water in the subducted slab and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids the isotope data indicate that not more than 15 of the sr and th in an average arc magma were derived from subducted material and that the rest were derived from the mantle wedge the fluxes of elements which cannot be characterized isotopically are more difficult to constrain but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab

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

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1